The way i see it is burst damage is more advantageous than firing arc. As a Defiant you should be "bobbing and weaving" so to speak when you engage enemies. As a cruiser it's fine to just orbit your target and spank away at them with beams, etc, but as a Defiant you need to do strafing runs...it's the main reason why i play the Defiant, it's fun to move and run around your targets. If you've watched the DS9 episodes where the Defiant engages targets it's very much this kind of strafe and burst damage type of play style. Of course you're free to play however you want but the Defiant is optimized for this type of battle strategy.
Add me: Ninja@MonCapitan
Keep in mind that the enemy is regenerating shields while you're turning back for another run. The quicker you get energy flowing back on target, the less shield is in place for your next cannon run.
Edit: also keep in mind the Defiant had beams as well as cannons.
Well the Defiant in the game is clearly not canon Defiant unfortunately. It's not even a "real cloak". I would really love to see the Achilles class starship
in the game. Actually if i could pick any ship to captain in the Star Trek Universe i'd pick the Achilles class.