I can see where Voice Chat may not be as necessary in an RTS environment, but I think that will depend on how large guild and alliance battles will be (EVE Online for example, where VC is ESSENTIAL during fleet battles). In a guild vs. guild battle where you may be coordinating with 20 or 30 actual players then VC would be extremely useful and I do think that 3rd party programs such as Vent or TS would be the best solution.
However, playing devil's advocate for a moment, 3rd party programs do provide an extra hurdle of technology for players and not everyone will navigate that hurdle with equal prowess. In-game VC would make it easily accessible to all players.
I think it may be something to keep on a sticky note for future consideration (after seeing the scope of battles, the actual use of complex tactics, etc.) rather than something that needs developer resources in the coming months.