Theres another issue what i haven't seen mentioned that is a usual complaint about In game VIOP services:

If your game crashes....you loose Coms. Infact you can't even tell people you crashed because....you crashed voice too. Its just not the best system. Sometimes the best functionality comes from not evening every function in one program/client.

Not to mention most VIOP "seems" shall we say, extreamly tacked on. Generally because it is either added later in the games life. Or decided to be done late in development.

Theres more pros and cons, but i'm at work so i'll have to stop here. ^^