To me, homebrew is like, well, "home brew": I don't want to do it, but I don't think there's anything inherently wrong with it if you're not getting out of control.
As for why I don't like it... well, I feel like it would sort of cheapen the game, make it feel more like a Lego set to be aimlessly toyed with, picked apart, and modified instead of appreciated as is, warts and all (I always made the exact thing the instructions were for and never changed it when I got those things as a kid... what does that tell you). I respect everyone else who wants to do it, though, and I think texture hacks and whatnot can produce some fabulous results.