One of the Soviet aces got shot down by the tail gunner of a Stuka, but for the most part his greatest value was as a second pair of eyes scanning the sky.
when you put a crewman facing the back and doing nothing else it means that you expect to get shot down. In the Mosquito DH say the crewman facing front, which signals "we are so fast we dont even need to look behind".
But seriously.
In a fighter, the gunner is just ballast 99% of the time. All the good day fighter made just fine with a single crewman.
For an attack plane, a second crew member is much more valueable, either as spotter or as defence. The cost in maneuverability is less significant to an attacker.
