All of these vendors want to own the instant messaging marketplace. Yahoo, however, are pretty stupid in doing this. They're probably number three, at best, in this market (and that's counting AOL and ICQ as one company), so they really can't afford to piss too many users off. Piss too many of 'em off and they'll just all decamp to MSN or AIM.
Next month, Microsoft plans to do similar things, changing their protocol in an attempt to knock out all the third-parties. I hear that most of the third-party groups already support the new protocol, so I won't be having any problems.
Thing is, this shit doesn't really bother me. I have accounts on all the major ones: AIM, ICQ, MSN and Yahoo. You can contact me on any of those. I don't do it with some integrated, upgrade-every-time-the-protocol-changes client. Instead I use Jabber, a standardised and free instant messaging system. It has server-side gateways to other networks, which keeps me in touch with people still using neanderthal systems.
So if you use Trillian, GAIM or one of these systems, let me recommend Jabber. Go get yourself one of the clients and stick me on your roster. My Jabber ID is the same as my email address. Enjoy the difference.