rares are actually great for dedicated bounty hunters too, who aren't concerned about the profitability of what is in their cargo bay. the fact is we *want* pirates to attack us, which they will do more readily if we are carrying something nice.
bounties are our bread and butter, so we can collect rares bit by bit, without the worry about there not being enough to earn a living. and when there's a full bay, we can sell them off for a nice cash bonus and begin the cycle again.
of course, a ton or two of plain ordinary gold works just as well to bait pirates...