I think you are asking the wrong question. A good DM is going to make changes to assure continuity within his campaign world. So, the question of altering gaming material, if neccessary, is very much the responsibility of the DM.
When I review a new sourcebook for possible addition into my campaign, the decision to add any portion of it is based on my sense of campaign style and flavor. More often then not, I find the material too, "far out," to use in my campaign. Many of the PrC's could make for a unique villain or group of villains, but, I usually find myself just stealing their abilities and feats and possibly adding it to my next villain, rather than taking the class idea as a whole. I often try out any new feats the PCs want on one of my villains before deciding if I will allow it for the PCs
As for improving published d20 material--I simply don't add what I don't need. WotC is just as guilty of putting out bad material as any d20 publisher. I think too many DM's are trying to add new prestige classes and feats rather than focusing on a comprehensive and believable world in which to find adventure. For them, the campaign world takes second place to the new materials coming out, and that, for me anyway, makes for a poorly put together campaign world. Keep it simple or you'll be flipping through yet another sourcebook or printout.
It's the ideas that count most! New d20 material doesn't mean it's better than what ideas you might already have brewing in your head. After all, publishers are in it for the money, while DM's are in it because they enjoy creating dynamic interactive worlds filled with adventure, action, and excitement.
Bottom line--don't buy it, steal it (the ideas that is).