Janitors provide no added benefit when on your ship. On the ship they simply hold a broom and walk around aimlessly.
Janitors who wield a dagger have no ranged weapons, so they must close the distance before they can begin to hurt enemies. This makes dagger-wielding janitors somewhat similar to mechanics (crew members who also lack ranged weapons, only being able to attack in melee range with their wrench).
Conversely, janitors who wield an assault rifle have no melee weapons, so they will not attempt to close the distance while their gun is on cooldown. This makes assault-rifle-wielding janitors somewhat similar to penguin mercenaries (crew members who alternate between two pistols, refusing to engage in melee combat).