That's certainly short-sighted on their part, but mercs suck up valuable exp. As you've probably read elsewhere, we hardly bother hiring them unless it's like near the endgame where you just want more bodies around. Even then they manage to die early in the confrontation.

This mission, uncompleteable or bugged? lol... many of us have cleared that entire sector out plenty of times without needing to hire mercs, even on hard mode. Plus, who says you need to fight them - you could always farm pirates elsewhere and use the loot gained to pay these guys off. Smart people will pick their battles wisely. It's the same like that guy who complained having always to mash the reload button and pray his team survives the next attempt. You don't learn anything that way.