CLockwerx: Trillian has plugin development support with an internal language that can expand the functions of Trillian in ways like mIRC.
Now, with that said, mIRC is an *IRC* client program. It doesn't need IM services, it's completely meaningless. In fact, multiple IM clients is a completely meaningless device. It causes problems, more confusion, and more waste.
But IRC was before IM services

so, ya know, it can stick around.
As of right now, AIM has the numbers, but MSN has the technology.