Well, I would like to add to this thread, simply because I’ve been asking for this since 4.0. It’s still amazing to me that nearly perfect engine like Unreal cannot do this very basic thing. I’m also coming from Cryengine and if you’ve used this technique before, it’s kinda hard to replace it with something else. Like, if you take Crytek’s assets from, say Crysis 2, you can see how extensively this is being used and how well this works. However, if you dig through Epic’s assets, it’s obvious that they rely more on mat blends and such.
But let me post an example task, which in my opinion is best solved with geometry decals. I would like to hear how would people do it without them:
- You have a huge network of custom city roads (so predicting a modular texture approach is not really an option), and you need to add road markers to them. Obviously, for variety markers need to have variable opacity and color, plus project them on underlying surface (so masked material is not ideal, we need decal kind of shader). Now, with CryEngine I put down a plane with tileable asphalt, then use any custom shaped geo decals on top, varying them with Vertex Color and Vertex Alpha. Feel the main difference here: if I need to make a curved dotted line and I only have a square Decal Actor, I need a custom decal texture. If I can use geometry, I take my tileable dotted line and map it on bent piece of geometry. This flexibility is just immense.
And there are just countless other cases which are just best solved this way.
I know it sounds confusing (if needed I can post images demonstrating what I mean), but hopefully Epic guys would give us this functionality one day 
Cheers!