Wow, I have absolutely no idea how to answer this...
I am the GM for my local group. I have set up three different campaigns for the group since 3e came out (plus a short-term joke campaign, but that is beside the point). In each campaign I laid down specific rules about what races, character classes, PrCs, weapons, magical items, spells, etc., were available. Once I did so, there were no changes (except for a couple of PrCs) allowed.
The thing is, though, that many time "core" ideas, classes, PrCs, etc., were nixed right off the top. In one campaign, for example, paladins were removed and replaced with the various classes out of The Book of the Righteous. In my current campaign we are using AU as a basis, so there are no elves, dwarves, halflings, etc.; neither are there Fighters, Clerics, etc., but there are classes and PrCs from outside of AU.
So, do I allow material from something other than the core books? Yes! Do I allow extra races and classes to be added into a campaign after the core material has been settled? Only very, very, very rarely.