I sometime like to punch some unsuspecting opponent (bots are great for this) with a Pirate Ship spite deck, and while I understand it's hardly ever the best strategy, it can be devastating if the opponent doesn't answer promptly.
I find that you need:
- trashing (most often, but sifting is also good)
- an engine on the board
- no or weak coins from actions (except pirate ship, that is)
- a collaborating opponent (that is, an opponent that realizes too late that money is bad for him)
The thing is, it's fun, it hurts, it punishes the guy badly, and leaves you with a strong deck afterwards (sometimes stupid good). Thief is bad for this because when you want to gear into greening, it doesn't provide much help, so you're not even sure you'll win in the aftermath.
All in all, I think Pirate ship (and Thief to a much lesser extent) are powerful attacks that are fortunately very easy to play around. The fact that PS was considered awesome for some time shows how hard it can kick ass on careless or unexperienced (or robotic) opponents.
I love pirates.