PC is a market, like 360 or PS3 is a market. There's no difference. Sure demographics are different, so what ? PS3 and 360 have different demographics too, but you won't call a PS3 / 360 game an 'exclusive'.
Well, it's the same for PC / 360 games, they're not 'exclusives' since they're available on 2 platforms.
If I have a PS3 / PC combo and decides to buy Mass Effect on PC because it's the best version, well guess what ? It makes one less sale on 360. Same thing when I buy a multiplatform game on my PS3.
MS earns money on 360 software sales, but it earns nothing when I buy a new PC game, or when I buy a PS3 game.
MS needs PC games to sell Windows though, so they can't kill completely the PC market. Hence the PC / 360 multiplatform games.