Having one party member save everyone else by retreating, as was the case in the alpha builds, is also open to abuse.
I don't know how feasible it would be to keep track of characters that flee combat, and then not let them telport back, or apply some kind of penalty for a couple turns if they rejoin the same fight (there shouldn't be a penalty if one person teleports to a co-op partner in combat, though).
Even with that, you could still have one character send potions and scrolls, etc, to another in combat.
Ultimately, the game was designed to allow that kind of use of the pyramids and magic pockets. In order to prevent this, you would probably have to set some kind of time limit on turns.