There should be an "Event End" which is generated by the server when the Session expires.
This would allow to analyse the value of variables which are counted through a session.
Just one example, but we already have some use cases: We count the numbers of purchases within a session. If we use the purchase event we would get the session counted for all lower numbers of purchases as well.
ie. we have three purchases -> on usage of the purchase event (or any other event) we would get 1, 2 and 3 as attributed values. If we have a session end event we woudl count to 3 and would only get 3 for that event and evar combination.