why you say ?
because a "good" player would rather build pirate coves firsts and ressources next.
I would go to the extent as to suggests that even if pirate coves don't gather ressources, it would be worth it to build a pirate cove on a ressource if it allows for 1 more pirate cove in the BFC of the city !
My only issue is that most of the times, due to the way distances are calculated, having the Cof a 1000 slums doesn't allow you for more pirate coves for the city.. max is generaly 4... sometimes you may build a 5th, but it is extremly rare and would mean razing your own evolved pirate ports, and building once them all once again...
Why not make it so that distance between pirate coves is "by sea-move" (cities and pirate coves counting as sea-move as boats can move there).. or so land tiles would count as 2 tiles for pirate coves distance calculation ?(may be overpowered.. but.. still, with pirate coves being only on coast and 2 tiles away from each others... many times it proves hard to even get 2 per cities in neighbouring cities ...