There is overwhelming evidence to show that piracy leads to an increase in sales. It doesn't make sense to dispute that anymore.
Given that taking copies doesn't cost anyone anything nor deprive anything from anyone, yet can lead to further contributions to society, it only makes sense.
Actually, if you look at the trends in music sales since mp3/napster arrived I'd say that on a macro level you have a very good case that piracy decreases sales. Also, if you look at the sales of top albums now vs. 10-20 years ago they have decreased significantly.
Sure, you could argue that this is because Britney and the gangsta rap-act of the week aren't as compelling as the high quality music made in earlier decades - but most of the content then was junk as well. We just happen to remember the good ones.
This doesn't mean that the music industry has acted sanely - their legal tactics are dubious, some of the suggested cures (like SOPA, and others before it) are much worse than disease and their "damages" are insane. And it has taken them a decade to go kicking and screaming into the future, with streaming music like spotify. But none of this means that piracy is good, and that a lot of people have downloaded music they would have paid for otherwise - alongside at lot of music they wouldn't have given second look, much less their cash.
It also doesn't mean that there aren't many artists who have used to their advantage - e.g. to get publicity - but overall it looks like a net loss. A big one.