The whole reason the others wanted Aaron in the first place was to study the fact that babies aren't born on the island or are fucked up so if he's born not on the island there's no reason for Ethan to steal him.
I'd be disappointed if that's all there was to it. They made a big deal about aaron early on when Claire went to see the psychic. Made us think there would be something special about aaron -- kinda like with Walt.