I have read, although unfortunately I don’t remember where, an interesting tidbit about this topic. There’s a logic to why certain bands (e.g. Metallica) dislike napster’s giving away of their music and other bands (e.g. Limp Bizcut) seem to like it.
Of course, it’s the money.
In general, most musicians have deals with their record companies which don’t give them a very large cut per album. Even top-selling groups who made it big recently tend to make most of their money from touring. Unless you’re a big name before you contract for that next album, you’re not going to get a lot of money per album from the publisher. For these bands, the publisher loses most of the money when piracy occurs. The extra unpaying listeners may attend concerts, promote radio airplay, and increase the fame of the band, as Myrr21 points out.
Established bands, who have been making high-selling records for many years, have a better bargaining position with record companies. As a rule, they make most of their money from commissions on sold albums. They also have the least to gain from Myrr21’s argument- very few of the people downloading the songs of an established group (e.g. Metallica) will have never heard the band before. There will still be some people who are curious whether this particular album is good or not, but the proportion of these to simple leeches is far different than downloaders of a garage band’s or even a recently popular group’s songs.
That’s one reason why the big-name bands dislike piracy. They have the most to lose and the least to gain from it.
That being said, I should point out that decades ago it was claimed that people taping off the radio would destroy the music industry, and that certainly didn’t happen. Also, there are legal reasons to act to defend copyright even if you fail to actually stop the violation.
(oh, and I own legal copies of all the Metallica albums, and 6 of TMBG’s albums. Go figure.)