Droids can be easily replaced, just go to a shop and buy components to make one
Humans, on the other hand, have to respawn at an undeathinator, so unless your ship is at a station where the human spawned at, good luck :P
However, I do agree that security bots should be able to be armed with heavier weapons than humans. After all, being robots they should have more strength than humans. Except, a heavily armored security bot with missiles and a plasma minigun is more expensive than a human, whereas a floating sphere with light armor and a laser would probably be cheaper than a human. So I guess security bots would vary in price, depending on how useful they are. A security robot with automatic weapons, missile launchers and heavy armor would probably be more useful than 2 humans in medium armor and laser rifles.