My thought was that they wouldn't have used the energy dampening weapon for the reason that they were then able to almost completely wipe out an unsuspecting, massed fleet, soon after the attack on Earth. The only reason anyone survived is that some were left to tell the tale and dispirit the Federation. Two severe blows to the Federation and its allies one after another.
I have to agree that the simplest answer is the correct one. It isn't a weapon to fend off an enemy, but to anihiliate one.