Having the ability to just talk with your party through text would be cool, although I don't think that should be the default chat option.
It would be nice for people like me who don't have a mic though and have to type to my party members and use the team-wide chat as a no-mic chat, effectively giving anyone else who is not in our party a sort of one sided conversation to look at.
If I'm duoing with someone I can use "/msg [username]" and then /r (after they reply to my first "/msg [username]" to whisper to them, but this doesn't work as well for parties of 3 or 4 people in a 5 person queue.