Trillian is periodically in an arms race with Yahoo -- Yahoo, which doesn't have ads in its own client, periodically changes its code so as to make their network inaccessible with Trillian.
I believe they assert this reduces their vulnerability to spamming. I fail to see how, but I don't know what other explanation there might be.
The most recent such incident occurred in June 2004. One user told CNet it was "Yahoo's attempt to stifle progress."
On the other side, the minor instant message spam (or "spim", sadly) problem has been compared to the minor e-mail spam problem of a decade or so ago. It may not be big now—I've not seen it—but apparently it's coming, and Yahoo wants to stop it before it's too late.
"Spammers are being aided by entities that are accessing our systems without our consent," said Yahoo spokeswoman Mary Osako.
A logical person might suggest Cerulean and Yahoo attempt to broker a deal, that is, to obtain consent. A miserly person such as myself can see the virtue in the current system, even if I use Trillian and my girlfriend in Nevada (I know, I know) is in practical terms only on Yahoo.
Since they do have ads, I would think it's AIM that would keep trying to pull this, but no. I don't use MSN Messenger, but I haven't seen indications that even Microsoft is trying this stunt.
In any case, Cerulean Studios promptly creates a patch for it, of course, and thenceforth all is right with the world.