[ralphm]

Presence subscriptions don't have to be symmetrical in XMPP, and indeed you can even have contacts on your roster that don't share presence in either direction. As for exchanging messages, as far as I know Google's implementation is the only one that doesn't allow that without presence subscription. All others do.

In fact, this caused presence subscription requests to be the only attack vector in terms of spam in Google talk, which some argue makes it harder to combat. Mostly because there is less data available to detect malicious behaviour.