Grogmonster, aircraft POH are written by lawyers for the lowest common denominator.
In this case that means the person who is likely to forget to turn an alternator on after starting.
By recommending it be turned on with the battery you preclude claims for compensation from the dingbat whose radios failed at the worst possible time leading to nervous shock and trauma.
That said, I have seen recommendations for both actions. One chief engineer reckoned the battery was the cheapest part of the engine start so don't risk damaging the alternators until after the engines are turning. Others just followed whatever the POH said.
I have yet to hear or read a convincing argument for either action.
