I don't think all skins require shading but I'm not a fan of large plain blocks of color on my skins. For me (and I'm sure most skinners) my skins start out with flat colors and then I add shading and highlights as needed. It's just part of the process. I don't feel like they are done til they are shaded.
I do prefer shaded skins over non-shaded but there are also a lot of different ways I've seen skins shaded and I'm not a fan of all of them. So whether or not a skin is shaded has no bearing on how well a skin may have been made.
As for if someone likes a skin or not, that all falls under each person's preference. So those that like shading will like shading, those that don't wont, and those that can appreciate both will do so. That's pretty much all there is to it.