Even if it weren't federated (though I do miss Google Talk), there would still be some value in using an open standard. If I can only afford a feature phone, there are XMPP clients for it (or they can be developed relatively easily), even if I don't get all the features. Using open standards, even if you don't make the service open, still makes you more inclusive and helps a lot of people who would otherwise be shut out of your service.