There's one way that doesn't need to be speculated about, because i see it happening often with friends:
People often use piracy as a "demo", and decide after sampling the product whether it's worth a purchase. So a common situation is to get an early cam rip of a movie, or an online-disabled version of a game, and then decide whether to go see it in the theater, or buy the full version.
Sometimes that kind of "demo" is not available though, so they'll just take the plunge and pay for it, finding out whether it's good or bad after the purchase, with no recourse available.
So, a leak is definitely bad for studios, because it'll deprive them of sales to the impatient. On the other hand it's good for consumers who would otherwise have wasted money.