I have dozens contacts in skype. They don't care if I use a skype client or what. They just "see me on" and ping me. Or I ping them. And then you probably talk on the phone.
Interoperability with skype at the IM level is great, because you could potentially talk to them with IM, and send a video request over google's new video over a browser thing
You just tell them "click here to do a video call", a client that talks through the skype network could do that