As someone I know would say if he were to post here, THE DEFIANT IS NOT A DD.
It is classed as a CL, and probably falls somewhere in between the DD and CL categories -- in SFB it would be a War Destroyer, which most would recognize is generally superior to a stock CL in firepower, but weaker in hull. That description fits the Defiant to a T.
The ship is an attrition unit -- cheap to build (in the game, that means cheap to buy and readily available at the shipyard) but packed with weapons. It is a perfect ship for those who like to be able to deal a lot of damage, fly fast, and out-maneuver their opponents, all while staying competitive with CA-sized ships at a fraction of the price. The only thing wrong with it is the other races don't really have an equivalent design.
I don't agree with its hardpoint layout, specifically having more heavy hardpoints than primary ones, but I gather Taldren is standing firm on this mistake (their decision). Otherwise, it is an ideal vessel for the role it should, and does, play.