Imagine that you're playing a 1-on-1, nobody places a bid, and the Pirates attack you first simply because you happened to be the player that they defaulted to. That doesn't contribute much to a level field if it's another player you're trying to match wits with.
Well i rarely see that happen, Usually there is some sort of struggle over bounty. Looking at the other comments here, valid points arise, and i agree that late even mid game pirates are mostly insignificant/annoyance but its early game actually the first raid that counts the most imo. If a player who for instance is cashfloating and building up their econ whilst putting very little resources into an early fleet is going up against a player who goes for a fast fleet + rush and hasnt got 2 cents to rub together, the cashfloating player is gonna win that bounty struggle and the rushing player is going to have to expend that much more resources (be it a starbase or keeping their fleet back to defend) to fend off that first pirate raid which buys the cashfloating econ player time.
I believe that there are currently some balance issues with particular races employing an OP rush strat or so ive heard (and appear to have witnessed)
vas rebels cough cough*
So i personally like the early game effect of pirates yarr
EDIT: also i have noticed that on small maps truce among rogues is useless without pirates. not worth the money by the time u have cleared the few non- asteroid/dwarf planet gravity wells there are no more neutral or at least un contested planets left