I don't have a problem with mages using lifetap...Just don't do it in combat.
Any good cleric will be watching his party's hp as it is his/her job to keep them alive
especially the mages hp.We know that mages are about as sturdy as broken glass so when your hp goes down we respond.Thus it is of the utmost importance to abstain from lifetap during combat.A cleric will usually turn away from the tank and to the mage...Tank dies...party dies...
Cheers