This is just going to mostly echo what markusin said above, but since pops accused me of being arrogant and pithy, I feel obligated to give a more substantial reply.
First, I can't speak for everyone, but my assessment of card strength not only accounts for what the card does when it's in your deck, but also how the card influences players' decisions. This mode of assessment is more intuitive for alt VP cards than it is for Actions, but I think I'd be grossly underestimating Pirate Ship if I ignored its psychological effect. Pirate Ship puts a time limit on just about any money-oriented strategy. Even if it turns out that on most Pirate Ship boards, Pirate Ship ends up never being used, but its mere presence forces all players to preemptively counter it, then Pirate Ship can't be shrugged off as a terribly weak card.
Second, Pirate Ship is still usable in engine mirrors. It just depends on [the kingdom] what else is there. If Copper trashing is possible but poor, Pirate Ship could still be a desirable card because it can be powered up to provide appreciable amounts of money. The rule of thumb that is often stated, "getting your Copper trashed is a good thing," doesn't properly qualify the extent to which this is actually a good thing, plus it makes the assumption that all decks can effortlessly transition their economies from 7 Copper to pure virtual coin. Sometimes the virtual coin is just not that good (e.g., terminal Silvers); sometimes the virtual coin is expensive (e.g., Bazaar); in those cases it could be very nice to have a card that hinders the opponent's economy in the short term and provides $3-$4 per play in the long term.
And sometimes your opponent decides that it's a good idea to use concentrated sources of Treasure as economy in a drawing engine. You'd be delighted to hit just one of his Gold.
tl;dr 2000th post and "lol Pirate Ship is so weak" is a bad heuristic. If we've learned anything about Dominion, it's that most blanket statements are bad.