Hm. On the surface, I don't have any problem with this, as long as they don't make it too "game-y." Case in point: Diablo 2. Fight boss, OMG I'm getting whupped on, portal back to town for healing and better armor / weapons, portal back, resume fight.
If you make an attempt on someone, when you come back, they should be better prepared.
You should also have to face the probability that you will need to take a different approach or strategy.