I was talking online recently with George Q, as we are wont to do. He was helping me test file transfer over MSN with my client and asked me about my choice of client. This got me pondering. I use an IM network called Jabber, rather than MSN, and George was asking why I used this. I have only a handful of contacts that use Jabber, and all of them have MSN accounts anyway, so why stick with this thing? It's not the feature set, since MSN has several features still missing from the Jabber protocol (eg VoIP for voice chatting) and it's not that the majority of my friends are using it, quite the reverse, as I mention above. The main reason, to be honest, is because the entire protocol is open source.
I do have an ideological thing about open source (although I still haven't made the switch to an open-source operating system) and that's what got me interested in Jabber. One (techy) thing that I like about Jabber is that it's entirely distributed. Rather than having a single (group of) servers concentrated in one place, anybody can run their own Jabber server and a user registers with one server. When they want to talk to someone who's registered on a different server, the client asks its server to get in touch with the contact's server and establishes a link between the two servers. This (theoretically) also makes Jabber a more robust protocol than something like MSN, since although a Bad Person could take out an individual server, the network would survive.
Also, because the protocol is open, anybody can write a server and client based on that protocol, and, indeed, there are several Jabber servers and clients available. I use Psi as my client and have set up my own private Jabber server using ejabberd.
So, I got into Jabber because I'm ideological and I'm sticking with it because I'm a stubborn old bugger :-).
Oh, and in case you're wondering, Jabber allows special server-plugins called transports which allow the server to see another (proprietary) network as if it was a Jabber server, which is how I'm able to talk to people on MSN.