There have been threads about this before. I will simply state some of the problems that accompany a voice chat system. It takes time to type a nasty comment about a team mate or enemy which may take a second to say through a voice chat system. This is the main problem and one of the main reasons why I personally do not want a voice chat system. There could be a mute function on this, however you may miss some actual useful information that the flaring team mate may say.
Another problem with this is the potential lag that the system may cause. In your post, you had spoken about the lag accompanied by Skype, this may very well occur with an internal voice chat system. It may be possible for efficient integration of this system to the game, however I personally do not know much about programming and I think it may be safe to say that this would be difficult.
This post may very well soon be flooded by comments like mine, however I think there are benefits to a voice chat system, however the true question is,"Is it truly beneficial?"