another aspect of federation is that it inherently requires giving up control. once your information crosses the threshold or boundary, it is outside of your control. so, how much of your information do you want to cross the boundary? maybe you're okay with providing your identification, but do you really want to provide your data too? what do you think other systems will do with that information?

with federated identity, you can get past gatekeepers and enter some venue. but the data stays in.