My opinion is my opinion, and therefore by definition cannot be incorrect. In MY opinion (which incidentally is shared by others in my professional working environment), is that it’s a lazy approach to development. It will prevent your game from standing out in a sea of carbon copies that share the same assets. Some mundane things can indeed be re-used, characters, monsters and various other centre-pieces cannot. Bought assets are great for prototyping, but I would never consider using them in a final build without considerable re-development towards my own tastes, unless it was absolutely perfect for the task in hand and I knew nobody else had done it already. There was a big argument a few years ago about a source-engine game using ‘stolen’ Half-Life textures, when actually it turned out that Valve used a texture pack they found online.
People WILL notice things shared between games. I’ve seen a tonne of games that use UDK’s default Sky for example. That puts me off enough not to buy it because it makes me believe the rest of the game will have equal lack of attention. I would also never consider employing a level designer for example, who couldn’t produce his or her own content, or an FX artist that couldn’t put together their own particle systems or sprite textures.
Also, if you have already spent time making original content, why buy a rock or a tree when you already have it? If the ones on marketplace are more improved than yours, then there are still things to learn IMO.
For those that only wish to program that’s fine, but a programmer alone will struggle to make a complete game on their own anyway. Art is part of the development pipeline, nobody can be a one-man army. Great for prototypes, possibly useful for learning, but unsuitable for real development is my opinion on the marketplace. If you’re able to make money from it, more power to you.