Pirates aren't particularly dangerous. If you have a proper defense waiting for them, they won't kill anything and in fact will level up your capital ships, and that's a big deal.
In singleplayer, the AI is rather careless with its units and gets the killed more often, and this means your capital ships level up much faster than they would in multiplayer (where a good player will retreat and repair and reuse his units rather than letting you kill them). The AI is also pretty poor at killing your capital ships, so you rarely lose them. The result is that it's easy to get high level capital ships and keep them in operation.
The opposite is true in multiplayer. Human opponents guard their units carefully and won't give you freebees, and they are much more ruthless when it comes to taking down your capital ships. Most capital ships won't live to see level 5. Every scrap of experience you can get to level up your capital ships is a big deal, and this means that getting attacked by the pirates is often a good thing for you.
As mentioned, pirates are more of a nuisance. They are not dangerous in any significant way and are easily defeated by a very moderate defense. As a result, most players won't even place bounty. If they're the target of the pirate raid, they don't care. It's annoying, but it's not dangerous and in the long-run you benefit from it.
