I don't think in-game voice chat is a good idea at all. It would have to be incredibly optimized and really good (better than the current options people are using outside of the game) and that sort of thing needs a lot of money and development. Not to mention the fact that they'd need to moderate and make sure that there is no illegal/against the TOS activity and that would be incredibly hard to do.

I'll give you an example. They added an in-game voice chat in World of Warcraft and it was terrible. Low quality and really laggy. People tried it once and never again.