1) Bullets, plasma cannons don't need them, they are fueled with mass and energy.Fire may me maintained without sacrificing your own mass to make bullets, stopping to reload, ot needing a supply chaing. 2) Armor penetration - not tomention the heat damages things a mecchanical shock won't. 3) Ditto for electromagnetic effect on circutry. 4) Cannon beats gun in general - size does matter. 5) People with functional plasma cannons don't get teased about their inadequacy...not more than once anyway.