IM gateways?

[Kurt Granroth]

Has anybody here used an IM gateway and, if so, what is their purpose?

I've been playing around with the OpenFire jabber server lately (very
slick, btw) and one of its features is an IM Gateway.  It allows you to
communicate with non-jabber IM servers (like AIM, Messenger, etc)
through this gateway.

As far as I can tell, though, you still have to register with the
"parent" server.  That is, if you want to communicate with an AIM user,
you'll still need to create an AIM account.

This is completely separate from jabber federation.  I can communication
with GTalk users just fine via my OpenFire server without having to
register with Google at all.

For the other services, though... what's the point?  If you have to
register anyway, why not just access those services directly?

Anybody know what I'm missing?
Kurt