First few thoughts.
It's a spaceship. The ball turret's weight is meaningless. In and out of gravity, it isn't using aerodynamics to fly. Much like a tie fighter. Not to mention metamerials and a lack of need for weighty ammunition or a gunner to man it.
Ugly is awesome. You want to talk ugly, lets look at the half eaten cheeseburger. The Millenium Falcon is ugly as sin. And that is its charm. It's a pirate ship designed to be heavily modifyable.
Why wouldn't the rebels try out a bomber with the same concept after seeing just how awesome the Falcon managed to be.
It's the same core concept minus the freighter. It's not sleek. It's not pretty. It's not cheap. But it can be made right for any number of scenarios and it does its job well.