That's why I also wrote this part: ; )
I'm aware of their purposefulness or not. But fact of the matter is, I'm the only mac at the lan parties (second best computer there overall), and I host all the games (stability is the main factor), so it is imperative that cross-platform be there. Now if I just wanted to play with myself (insert immature jokes here) or with random people (I REALLY hate random people), the sure whatever. But personally, I'd much rather play with my friends (insert more dirty jokes here).
On the other hand, there are those who would like to do multiplayer with their PC-using friends (not necessarily in the GameSpy-type forums), who simply will find it won't be useful for them to buy the game, if all they have are PC-using friends to play the game with. That's an understandable decision -- is that perhaps what you meant, as opposed to "boycott"?