Oh, to be sure, I'm not completely draconian about it. I've let in rules, feats, classes, prestige classes, spells, etc. from other sources after review (I've also said -no- more than a few times). I just let people understand up front it's the exception not the rule so no one new to the group is disappointed. Anytime you say "let's play DnD" there is an understandably large number of players/DMs that take this to mean "lets haul out our library of official, non-official, and/or fly by night publishers d20 books and play D(ewey) and D(ecimal) the RPG."
I've always played that way really, I'm the TSR/WotC nightmare customer. I need a PHB, the MM and I'm good for a multi-year campaign (the DMG for whatever edition is nice, but optional - I've already got the Gygax DMG always on hand, heh. It's quirky and flat out bizarre but somehow packs more inspiration in it's pages than most of the newer texts.)
The only reason I don't have restrictions on rules like this is because I know my players. Otherwise I'd cut stuff down to settings-appropriate. Only time I've ever had powergamers it was in deathmatch campaigns that were a bit more adversarial. (Build fairly/within the rules, play to win, on both sides.) Well, there was that one time, but he learned he can't out powergame the GM....

Generally, I do pick up a fair few of the GM suppliments when I can for two reasons, one, is there's some neat stuff hidden in there, and I can twist into whatever campaign I'm currently running. Two, I may as well vote with my wallet if I like something, right?
(Incidentally, my GM resources aren't limited to the system I'm using, either. I've stolen a ton of things from Pathfinder and 2e Dark Sun for my current 4e game.)