^He indirectly replied to myyke, I'm guessing.
Why would they? They're big boys now, they can handle the franchise now without running back to Bungie for every little nostalgic reason.
It's not unheard of. EA went and had Frank Klepacki come in to do the Hell March for Red Alert 3 even though he had moved on with the other former Westwood guys to Petroglyph.
The other VAs aren't Bungie employees in the middle of their Next Big Thing (tm). It's also easier to replicate high pitched alien squeaks, hoots, whistles and one liners than the identifiable voice overs that comprise the main characters.
I'd disagree. Staten's voice is pretty identifiable. I can definitely tell which grunts are him in Halo 3, and the Super is basically Staten's normal voice with a bit of a filter.