The issue with this is there would need to be some new flag added to boss events... otherwise it would break backward compatability with every other level that has a boss. Also, how would this work? The boss activates, you kill it... and then what? If the level doesn't end, does the player just move to the next area? In that case, can't they just run past the boss entirely? I can see one use for this: having a boss fight where you have to kill a man boss (say devan shell) and he's backed up by other bosses, which can be killed but don't end the fight.

Other than that, it wouldn't really do much. Maybe if the boss event was tied to a trigger somehow, and when it dies the trigger is activated? That would at least force the player to fight.