Analysts believe Microsoft and Yahoo don't want third-party clients on their networks because they use their own clients to deliver advertisements and direct users to other services. "Both Microsoft and Yahoo value the control over the clients and the last thing they want is for their users to be using third-party clients on their networks," said Michael Gartenberg, a research director with Jupiter Research.
(Yes, they have a Linux client; but I'm not going to download it and give Yahoo my bandwidth to blast ads at me. Bugger that for a game of soldiers. If you want to reach me, use Jabber; my ICQ or AIM accounts will still work, at least until AOLTW lock those down.)