Well, it should be! 
The other obvious advantage (and I do this constantly while developing) is to go and see what a particular function actually does in the engine. I may not always understand it of course, but it’s so much better than being limited by whatever API documentation exists. Not to mention I can just do a solution-wide search (find references) and see if that functionality has been implemented in other places to learn how Epic uses it. It’s very empowering.
+1.
I probably wouldn’t be able to figure out how to extend editor if I didn’t have access to sources.
But, because we have sources I could check how parts of editor are implemented, and reverse engineer it.
+1
source is a must.
If documentation is a limitation, then it’s a fault of the engine developers. Not ours, tons of games have been released from the likes of Unity without any source code. Hell games have been released in CE without source, if you can’t develop a game without it then there is a serious problem somewhere. Whether it be the developers or the users…
An engine is a set of tools to make games, not a set of tools I have to work on anyway to make games…
It’d be interesting to see who is actually making games commercially here, or just for fun even. Different situations affect different opinions obviously…
I think ideally you want both great documentation and source. Since the former is usually lacking because nobody wants to write it and keep it up to date, the latter can help immensely. Case in point: http://docs.unity3d.com/ScriptReference/Media.MediaState.html
Took me all of 2 minutes of random clicking.
It’s not about not being able to develop without source, it is about how the availability of said code can be a great advantage. There are lot of situations where interacting directly with the engine code can be an advantage in many ways. Performance included.
To say that the requirement for engine source is a symptom of “something wrong” seems uninformed. A lot of triple-A studios surely make use of engine source when licensing engines. I imagine that is why source inclusion used to be the norm until the indie wave hit…
It is a requirement of “something wrong” and to think otherwise is “uniformed”, we don’t live in a perfect world and Epic or any other engine developer can’t always cater to every need. For example, we worked on a hack and slash project we built additional tools with the API in Unity and there were no problems. Because it was within the limitations of the engine, why in that instance would I need source code? Now our current game is an open world RPG, the engine won’t support it and we constantly ran into limitations, at that point we needed source code. Unity say they can cater to AAA, but I found that for some instances not to be the case. Is it our fault Unity has limitations? No, it isn’t but that shouldn’t stop us from being able to finish what we started.
So people say you “need” source code, maybe / maybe not like everything else it’s circumstantial, it shows if you’ve actually worked for a AAA company. A lot of times they licensed engines for political reasons, not for benefits so they can play the blame game. Also a lot of “AAA” in APAC are using Unity with no source code and again many of them decided to bypass that whole debacle and develop there own engines in-house for many reasons.
Although I agree with Giantoad, Ideally you want a good mix. But remember indies and other developers have been releasing games for many years without it, a lot of it comes down to experience.
You twist this towards a discussion of skills. It is very appearant that your own skills are an important subject to you:)
To discuss to this length wether or not source is needed if you have skillz - when this is not the subject - clearly proves my point.
Doesn’t really prove anything, you can’t deny that many games have been created without source code. Were looking at facts here, if source is important to you fine I’ve never said once it’s a bad thing to have. I keep saying it’s very useful, but an outright necessity, no…
I’m not sure what you’re arguing against? It’s a good thing to have.
Your posts mostly emphasized how a programmer with skill, or one who didn’t suffer from “a problem somewhere”, did not need source.
That was what I was arguing against. What I was arguing for, is that source is an important addition to the package for me, and many others like me 
Well there seems to be a lack of understanding as to what you and many others like you do exactly? From your other post it seems you’re learning the engine from source code, which is great. But the general use case scenario of an engine is to make games… So yes if you want to be an engine developer and learn how AAA makes engines, I’m pretty sure Unity would be a dead loss for you.
But for people making games, Unity can be a great tool. I don’t think Unity is quite as bad as some make it out to be, neither is it great either…
And to be able to fully utilize the engines capabilities, according to these use cases, source is a valuable asset.
Everything you say that you can do without source in Unity, could probably be done better with source.
To remake things, in stead of improving on the things that are already there, is counter-productive and in most cases a bad move, performance-wise. This is the daily life of a Unity-developer: To remake things, already in the engine, because what is there is incomplete or lacking. Pathfinding and GUI are good examples of this…
If you could improve directly on the engine source, you would also be able to program in c++ and circumvent the whole mono-package. This could solve many problems for unity devlopers, where mono is seriously outdated.
So do you think you can do a better job than the 120+ staff at Epic or the 500 staff at Unity? You say “improve directly on the engine source” so nonchalant, we were re-making things in Unity and it’s one of THE main reasons we left Unity. Because I have a team of 15 behind me and it costs a fortune whenever they spend time doing things outside the scope of the project. When a publisher is about to kick you across the room because you’re way behind on targets due to problems, then source is …
But I know what’s involved in the rest, trying to balance making a decent game whilst improving an engine is crazy and requires a lot of man power to do so. If it requires that many upgrades in the first place, it’s probably not the ideal engine to be using aye?
I wish it worked like you say it does in the commercial world, anything we needed we could just pop in sort it all via “de sauce codez” save the planet and be home in time for lunch.
Do you think that YOU can make something better than all the Unity devs?
Or have you never reworked anything already implemented in the engine?
How did you solve GUI?
It is about being able to customize the framework to your own need, not about adapting your needs to the limitations of the engine you are using. If you can not see the importance of this, I seriously doubt your creative zeal.
I used to be a full time engine developer, so probably in some instances in many no. How did you solve GUI? Doesn’t even make any sense, you can doubt peoples creative zeal until the cows come home. But what matters is the money keeping you and your company a float…
If you believe that myself and many other developers are incorrect, show me your game… I’d be interested to see this amazing creation with a re-work of UE4’s framework.
No need to be personal.
This whole post, however, just adds to the foundation of my previous posts: You appear more intent on promoting your own skills (and everyone elses appearant lack of skills), than on constructive discussion…
The importance of my status as someone who is “only learning the engine”, also seems to be important to you. If you were the triple-A engine dev you are claiming to be, I have to wonder why you suffer from such obvious inferiority conplexes conserning your own skill.
It’s nothing to do with promoting skills, mine or anyone else’s personal skills are no benefit when you’re working on your own project. We just need to look at both sides of the coin that’s all, I like source, I want source but you never answered the original question. Is it necessary for all circumstances?
I’ll apologise if I’m coming across personal, not my intent.
I have never claimed that it is an absolute necessity for all. I have said it is to me, and many else, and probably a useful addition to others. You are the one arguing for a universal truth in this matter: The truth that any (skillfull) developer will be completely fine without it.
I abandoned Unity because of the lack of long ints and the way quaternions are handled. In great cotntrast to most convertites, who probably do so for graphical reasons, I might add.
With engine source access, however, all these things could have been adressed and a change of engines would be unneseccary (not that I regret the move…)
EDIT: I mean double floats, not long ints.
Source code is not just niceties… nor is a must… but it is very important should you need to understand the inner-workings of the engine. Or if you are currently stucked with binaries, and if the source is available it can be tweaked to your need.
Not to sound condescending, but if you have completed and ship something (games/simulator), then you will understand the importance.