1) they are able (forced to) handle spam for Gmail (and internally for gtalk) - I'm not convinced by the spam argument.
2) there were only two "big" players, Google and Facebook. The benefit of federation would be as with email: an open internet with federation across organisation level services (community/company run servers).
The fact that Google didn't implement ssl for federation doesn't mean people "wouldn't" federate with them, it mean Google didn't make a real effort.