The most USELESS monster.

No, pirates are quite good for their first level, especially on the sea. They move quickly around the ship, and if you also cast a speed spell on them (by the way, it's the most common spell, and scrolls are often found), or something like Bloodlust... The only downsides are: 1) they're not useful on land, and there aren't that many sea battles; 2) they can also lower morale if you're playing a good faction.