Plasma weapons are usually frowned upon, because they lack pure power or pure strength, and especially the rifle because it cannot be overcharged, lessening even its ability to handle shields. Even more, its shots are bright, giving away your position. So why would you ever care for them?
The answer? Because of some perks that beginners that look for pure power overlook. Most beginners look for rocket launchers. It has lock-on, power, and does not arc like the wraith cannon does. BUT, and I can't say this enough, BUT it is EXTREMELY RARE, and could end up with you pumped full of steel by the competition in multiplayer. The plasma rifle, however, can be found in both bases usually, and is carried by many elites. Plus, you can dual wield, and although people may frown on its rate of fire, you can twitch your finger on the trigger to increase your rate of fire. It is also more powerful shot-for-shot than the SMG, and so potshots will do more damage. And the weapon has a better accuracy as well, a little known fact is that the plasma rifle is a fairly powerful weapon against hunters, trumping most other common weapons that you might find. Even if it doesn't do well in destroying plain armor, the sudden and severe drop in shield strength will have your enemies think twice about going up against you.