One of the commenters calls it out as feature ICQ has had for 10 years, but for me, it goes even farther back -- to 1993, using talk on an NYU shell account to message my friend Dave down in North Carolina.
It was very exciting and very futuristic. We were easily impressed, back then.
Of course, since there were no online presence indicators (for us regular lusers, that is), if you wanted to talk to someone, you had to keep fingering them until you found them online.
I always found the resulting conversations to be very stream of consciousness, since going back to change a word or phrase you'd started would only slow you down. It also gave you a very real and very powerful incentive to improve your typing.
As to why include it in the next AIM client? I'm not sure -- it seems gimmicky at this point, and I think there are better reasons to initiate a direct client-to-client connection.
Unless you're a typing tutor, of course.