I think pirates should attack all players that have bounty on not only highest one and with force proportional to the bounty players have on.
I think this would be a great solution, maybe the person with the most bounty gets a bonus of a couple ships sent after him, but if one person has 10000 credits on his head and another has 10001, it doesn't make sense for the guy with the extra 1 to be swarmed by bounty hunting pirate and the guy that still has 10000 on his head to be ignored
I think the pirates are good XP for cap ships.
This is my current view of pirates too. In the games i have played against both AI and a friend of mine (i admit i haven't had much online experience because i like the exceedingly long games) the first pirate wave is pretty much the only threat, and if it ends up being too much we both just abandon the planet for ten minutes or so and let the pirates have their fun while we concentrate on taking other planets to make up for the loss. The biggest impact they have had was when i coordinated an attack on my friend with the first pirate attack of the game to force him to loose a planet.
My only complaint about the pirates as of rebellion beta is that the pirate bases are actually a little weak. I tend to rush new stuff and big ships, so i had a titan out in 45 minutes. after two pirate attacks the titan was at level 4 and i had a couple of hull researches done and that was it and i decided to test the strength of the titan. I sent the titan to attack the pirate base to see how long it would last. That titan took out the entire fleet that was defending the pirate base in one shot. it didn't make it out unscathed by any means, it had around 4000 health when it was over, but i think being able to wipe out the pirates main defenses with one ship, that fast is not right.