Theres very few aftermarket batteries that supply a vent hole, they are all usually sealed
Ive seen a few cars come in with aftermarket batteris and the vent hose isnt connected. Yes dealer batteries cost a bit more but they are OEM replacement with a 2 yr parts warranty. If price is the factor a aftermarket battery will work w/o the vent hose but if the battery damaged your trunk area BMW will not cover it.