No they don't. That was sort-of true 3 years ago. Pirated content of current releases is from camcorder recordings. For instance that's all that's available for Transformers. Picture quality is generally shit. This still from the top-rated download may look OK, but in reality as a movie I find them pretty much unwatchable. And that's better than most camcorder recordings -Transformers was very popular around the world, and has been out for a while already, so better camcorder recordings are available.
Yeah I'm able to believe bootlegs are a slight positive, because anybody who wants to see the actual thing enough to suffer through a low-quality bootleg is going to want to go and see the movie in a theater.
Of course where bootlegs hurt the studios is in Blu-Ray recordings, where easily-available, free, high quality versions of the movie compete with the same thing that is not free. There is no economic method for not-free to compete with free. Just legal threats and an appeal to morals.
One exception is after the Oscar nominations in February or so, where review screeners are bootlegged, and some art movies may still be playing in the theaters.