I would restrict the trait for bigger ships only - those for whose "instead of shooting any weapons" is really a cost. Installing plasma mothball (2) on patrol level ship would make him real fighter killer while working as close escort for big capital ships. Could become quite unballanced.
I intend the mothball to be some kind of last chance to get you ship out of an unpleasent situation. Make All Power to Engines, activate the trait and hope you'll get out. Installed on a battle level ship it won't prevent the ship from being target for fighters - it will just keep them at low to medium numbers - noone will give up 15 AD Heavy Plasma Cannon shot to kill a single Frazi.