1 word. Communication.
In many more words:
You can't communicate for crap via a mouse, or controller where as a keyboard would be much faster.
A keyboard can arguably be better than mics since game noise can blot out people speaking, and shoddy mics, or, dare I even bring it up, their voice (I'm self conscious about my voice :/) can be misinterpreted, or choppy, or something where as text would be clearly readable. Further keyboards are generally very reliable.
Honestly, I think every console player should have at least a cheap plug and play usb keyboard with them that's compatible with their consoles. Instant messaging/mail/what ever you wanna call the text based communications is on both PS3, and 360. Playstation has "Home" which is sort of an MMO where text communication is prevailent.
The last thing I want to see is someone half-assing trying to talk to me in text. I.E. "ho want to party" (Considering since I'm a female it can sound like the dude was caling me a "ho.") as opposed to "Who wants to party?"
Frankly I think all multiplayer games should have a text chat system built in that can be seen easily on a standard definition tv of at least 30 inches, regardless of it being pc or consoles, too, but that's not on topic.
So, yeah, get a keyboard to communicate with.
As a gaming tool it's indespencible!
A keyboard as a controller, however, meh! lol
I'll end off on saying this is the PoV of someone who doesn't want to play on PC. I could elaborate more, but I don't want to go off topic, and I've rambled enough.