Right now we have more Assaults, Ronnie, BoB, Balls, and a backup. Our Heavies are just Psy and Shun.
But BoB has been using Assault Rifles most of the time, so he's already equipped with plasma.
Heavies have the lowest accuracy of all the classes in the game, and tend to be used more as support soldiers, and since their support effects (rockets, holo-targeting, suppression) work regardless of the weapon damage, its probably better to pick up the alloy cannons first. Suppression and Mayhem is the biggest benefit to picking up Heavy Plasma, and we don't have someone who can use that combo yet.