Well an unmanned ship could be made smaller than a manned one. They'd basically be phaser emitters and torpedo launchers on a warp engine. And the Cardassians did use them to great effect in the Battle of Chin'toka. Squadrons of such ships could have a place in swarming on the more heavily armed ships allowing the bigger maned ships to hit easier targets and cause heavy damage to the Dominion Cardassian fleet. Until a Founder breaks into Starfleet Command and reprograms the drones.
Not to mention that starships in the Star Trek Universe seem to be equiped with larger weapons that are more powerful to use against other large ships. When swarmed with smaller, faster drones it would be akin to swatting mosquitos with a baseball bat. Whichever mosquitos you hit will be out of play but its a terribly inefficient way to deal with large numbers of drones as you are taking significant damage.
It would be like building an APC with a TOW weapon system as its only armament. That might work great against other APCs, Tanks and buildings but would be terribly inefective against opposing infantry.