IRL an MG81 or MG81z (3200rpm) shooting at your face could really ruin your day. In virtual skies we do not need to consider how deep we are on enemy territory when an oil line or a fuel line has been cut by a single lucky bullet. I don't recall e.g. Rudel ever to consider his tail gunner as unnecessary weight. He was a valuable extra pair of eyes and if he was skilled he could direct the pilot to dodge the aircraft away from attacks and with his gun he could prevent the enemy fighters from getting into easy kill angles.
So even if you could get the impression in AH that the tail gunner had a very insignificant role IRL they were an asset, enough that to be included in newer zerstörers, too, i.e. ME410.
I'm not sure if they were useful in nightfighters, though. Mossies and Beaus having four cannons and because of darkness the intercepting nightfighters were hard to see to begin defense in time -maybe enough time just to scream to pilot that we are going to die in a second...
-C+