Indeed - clearly not a weapon, and not of any value in a more conventional fight. Otherwise, Locutus would not have been confused by it!
One wonders how it was implemented, though. The fireworks seem to come from the dorsal phaser strip, but no technobabble associates antimatter (or the ability to produce antimatter) with phaser strips...
Still, I guess strange stuff coming out of a phaser is better than phaser beams coming out of strange locations. And some episodes describe phasers as transporter-like devices, capable of carrying substances or even devices in the beam (see "Macrocosm"). Perhaps the dorsal phaser strip can transport antimatter and other surprising substances into space to strange, harmless but sometimes tactically advantageous effect.