There's no necessity or moral right behind it, but I fail to see how an action which does not harm others but benefits yourself is immoral --- much piracy has this form; here's a study which says that only 20% of album sales displace sales: http://www.nber.org/papers/w10874.pdf?new_window=1
Not to mention the almost impossible to measure fact that once a work is in the zeitgeist, by wide spread sharing, it also will likely make more money than the same song that didn't get that level of exposure with no piracy.
If this wasn't true, why would the music industry pay to get songs played on the radio?
