Gundato: Again, you are missing the point. You are under the assumption that Mac users will "degrade" things for us. The only argument I can see is that users in general will be detrimental to us. So if your argument is that it is bad for PC gaming for there to be more PC gamers, say so. Otherwise, explain how Mac gamers in particular can hurt us.
Honestly, the only issue I consider significant is the growth of Steamworks-like services on the PC. As Steam grows (in this case, expanding to a new platform), so will the likelihood of publishers using Steamworks (which forces users to run Steam to play games, even if the user does not purchase the game on Steam) or creating their own, similar systems (Ubisoft's recent DRM/community features system).
But hey, it's all okay because Valve can do no wrong. I am troubled that other users are comfortable with Valve's "you will run our store at all times, and you have no choice in the matter" attitude. Since when are people okay with bundleware, background applications, and mandatory storefronts? Steam itself is acceptable because users are free not to use it. However, users have no choice regarding Steamworks titles. If a user wishes to play Dawn of War II, for example, he must install Steam and run it in order to access the game.
Thus, as Steam expands onto Macs, its influence will grow and its compulsory client system will likely be adopted by more publishers. Once again, how do we, as gamers, benefit from this? Console gamers are perfectly comfortable with central planning. As PC gamers, are we not different because we want freedom of choice? If so, why are we eager to forfeit it to Valve et al?