The Steam's in-built chat always was miserable to use for anything other than messing around.
Sure, it's convenient, but that's about where the pro's of it end.
It's also very prone to lag, have horrible quality, and tends to break the overlay (not sure if that one's just me, or not). That's with like 3 different people. Imagine 6 more.
EDIT: Oh, and the part where "It's on steam servers, so we don't need a server for it like we do in Mumble!" Well, I've had my mumble up and running for like a year now, so it isn't like I rented it just for this. In other words, this isn't even a problem.