I always revive when i can... unless I suddenly find myself without gems (happens) or if we are next to a keep that is not flagged (please release and save me the gem).
Oh but the worst are the people that instead of rezzing you they steal your seige you have been using
Edit... I also forgot to mention that when i bring my low level characters to cyrodiil i often forget I need lev 50 gems.... I am sure this happens with other people as well.