I stay logged into gtalk on my Nexus One continuously. I don't notice any significant battery drain because of it, and I often have longish conversations with people while waiting for the train via gtalk. The gtalk app doesn't even show up as a significant user of battery power in the "Battery Use" list.
Actually, from talking with the OneSocialWeb [1] and BuddyCloud [2] guys, I understood that Android phones have a really sophisticated "always connected" philosophy. The main CPU can go into deep sleep and the phone still maintains a persistent data connection, waking up the CPU on incoming data. That's for the low-level IP stuff. TCP-over-GSM really sucks your battery.
XMPP doesn't (yet) have a standardized way of filtering data. Using Privacy Lists is a workable hack. Google implemented their own thing (using Protocol Buffers [3] to replace XML-on-the-wire, instead of gzipping as the XSF recommends). Process One has their own system as well for OneTeam [4].
The future ideal is to use SIFT [5], but it is still experimental and as far as I know, only Prosody [6] has partially implemented it.
The reason is that Google has control over the XMPP server. They do not need to send you presence notification if your gtalk client is not in foreground. Just staying online is not the problem. The problem is that usually with XMPP the server sends presence asynchronously. There is no standardized way for a client to request not being send those. If you happen to have 200 or more users in your roster there is always someone going away or coming back. Receiving those packets is draining the battery.
If your XMPP server happens to support privacy lists you can request not being send those presence notifications for a while. But at least some servers do not send the last presence when you clear that privacy list. And at least some of those servers do not allow you to get presence for all users in your roster by sending only one (unaddressed) request for presence. You have either to send all users in your roster that request or drop the connection and reconnect to get the current presence of all users.
