well, the defiant wasent really a BAD idea....its just that it was too easy to catch on to.
if you think about it, most fighters had there guns pointing towards the FRONT. and when your trying to dogfight your trying to get on your enemies SIX.
The defiant basically created with a "haha fooled ya!" idea in mind.which at first it gave a nasty surprise to those used to fighters shooting from the front and not from the back.
but, it became easy to catch on to. and thats why i think the defiant failed.
i DO think the defiant would of been good as a bomber attacking.
just replace those 4 .303s with say, 2 .50cals or maybe even a 20mm, then fly under a bombers wing and just shoot its wings off from underneath, safely out of reach of the bombers defensive guns.