Voting for example, still needs to verify the user session, and validate the node being voted on and retrieve the new vote count. This could add up to nearly the equivalent server load of the existing method or rendering the entire site view. Then you'd have to make additional AJAX calls to update all the other bits that come with the entire site view (cb, messages, xp). So now you've split one request, with one overhead, into several requests, each adding their own overhead. Now factor in that you can vote for several nodes in a single request without AJAX, but you have to make a new request for each vote with AJAX and you've multiplied the server load many times over yet again.

you don't know that of which you speak. doing a vote update does not require the cb to update. updating the cb doesn't require the votes to update. think man, THINK.