I don't see why. When the strategy is so easy, what's the harm? On small maps, sure, you would almost have to stop progress to do it, but on large maps, it's nice. Since you usually have multiple pirate bases or one easily accessible one near the center, it'd be easy to get at least one with an hour or more of gametime left.
I thought the same thing, why not do it if it is so easy. I don't want to speak for others but I am interpreting the point as you do not need to conquer the pirates to win the game. To me they are another dangerous ai race and a loose end that should be tied up by the end of the game. It would never dawn on me to quit the game before they are brought to justice, but it is a deep game and everyone enjoys it differently.
To me its like destroying the Antarans in Moo2, you don't need to inorder to win the game. but why wouldn't you?