In direct combat, you're absolutely right. Most of my point is that they aren't hired to defend most ships if companies do the math and assume the risk isn't worth the cost. The crew that's left are trained to fix the engine, cook some food, and control the auto pilot, not to fire guns.

That said, when mercenaries are defending a ship, it's often trying to stop a small runaway boat loaded with explosives. It's a very small moving target they have to hit with little time. Meanwhile the small boat just needs to be pointed somewhere in the direction of the oil tanker.