But what about the effect on the other person talking to the Supertasker. They are the real victims here, since I'm assuming that the Supertasker is concentrating on the road more than their conversation!

For me, the best feature of any IM client is GAIM's ability to do as-you-type spell check. When I'm just talking with friends, its no big deal, but with co-workers I like to be able to have a little more professional conversations.