Wouldn't the gaps left between the maniples actually mean the barbarian enemy could just swarm into the gaps and outflank every maniple instantly and hit them from three sides if not all four at once, if the principes were not immediately behind the hastati?
That works both ways though, because if an enemy entered those gaps, the Romans could hit them from 3 sides too. I imagine a barrage of pila coming from 3 different sides would be quite devastating