some of us don't live and die by playing only in matches ever. if they simply added teamspeak and all other apps to a banned list then public servers wouldn't be full of cheaters. but yes also in matches it can be used. while a guy playing might have his hands full fighting 3 guys yelling " i'll take you all on" someone on TS can say where he is, if the whole teams there, check how many are left, can he see them all, say one area is definately clear because of that etc, etc, etc, aswell as obviously if they've run into the whole team going to plant a bomb at B and everyone else is at A they can tell them.
its more limited use, but its mostly on public and even private servers you can find where Clans simply cheat all day long. i can log on now and go into a "noob" server and you'll have one guy at 75-2 and the rest sucking, or you can go to a semi decent uk server with lots of clan players who are all cheating. its just no fun to log on for a quick game anymore.
banning all external voice chat simply gets rid of any even chance of ghosting and cheating. the ingame speak works 100% fine, not had a single issue with it, its fine for clan matches and is fine for public servers.
i can't remember, at last count i joined about 7 uk clans and quit them all after a couple days as i just don't want to cheat to win, it sucks all the fun out of the game for me.