Splat books hurt 3rd edition more than they helped it. Initially they were all pretty good but as WOTC started reaching more and more into the clouds they got really out there. I quit DM'ing for awhile because I got tired of arguing with players who wanted to bring in nutjob things from the books. The one that always sticks in my head, probably because it was with a close friend was the argument that came close to blows over whether or not his "kit" first level barbarian out of the barbarian's handbook could have the 10hd battle cat. To this day saying "WOTC said so, takes any player right to the edge of no game at all."When I first played it it was my favourite edition, but the more I play it the less I actually like it. Some of the things at first feel good, but I believe in my case it was only that the lack of fiddly things was refreshing coming from 3.5 and PF1.
The system was allowed to get stagnant. They never did anything with it. Pretty much no books. No new classes. No Tome of Battle. Nothing. Half hearted campaign setting books that wasted pages on unwanted adventures. What happened to pure campaign setting books? What happened to class splat-books?
The biggest problem with any system is every new book introduces some new change to rules. Everyone that gets the rules to theplace they want them is then forced to stay behind, go to a new system, or do what they don't want to do. I'd much rather see modules and adventures than splat books. Players often ask for stuff they find in them but they don't feel entitled to them like they do with a "rules supplement".