I agree with the notion that game development is much more complex than just the very mechanics, and generally one should strive to cover as many aspects of that as possible. It doesn't necessarily mean that someone must excel at visuals, audio, storytelling etc., but get at least skilled enough that the game doesn't feel like it has major lacks.
Especially when it comes to art (since in the Jam/competitive conditions audio, especially music, is obtained from free resources), I believe making the game aesthetically pleasing is well within most people's capabilities if they try and learn a bit. Not every art must be "drawn" per se - if one gives it a bit of thought, they can get away with assembling rounds, rectangles and/or triangles together, maybe taking some time to choose different colours than the standard palette. Plus, there's more to visuals than just pictures - one can prettify the game by well-placed particle effects or procedural animation (bouncing, growing, shaking, waddling etc.) which, in fact, is more of an engineering challenge than artistic talents.
I think there's much bigger gap between "ugly" and "acceptable" than between "acceptable" and "visually gorgeous", when it comes to how it affects people's judgment of the game. And the former gap is easier to cover, too. Once you learn to make "acceptable" visuals, you are free to excel at other areas (mechanics design, storytelling etc.) without having them brought down by appalling graphics.
I don't know how about others, but if I were stuck with specific artistic/sound resources, I'd feel way more limited than I'd like to be. If I want to make a spooky game, I just want to crudely draw my own bedsheet-clad individual rather than dig through the preset resources to find out there is none. And even if some resources gave me a game idea, it might turn out it requires other resources, that aren't in the pack.
The resources limitation might work when it comes to non-game (tutorial/engine) challenges, so that people pay attention to the engine rather than placeholders used. But for a game, I'm not quite fond of that.