I do purchase alot of music (I hate purchasing downloads) on CD and Vinyl (and if it's unavailable on either format - cassette)
Usually my reasons for downloading via Limewire are one of these seven:
1. The album/song is out of print or the record label went out of business, making the album commercially unavailable
2. The album/song is unavailable in my area (for example, the Japanese release of Favourite Worst Nightmare by Arctic Monkeys contains two songs not on any other version. Their label later put out a 7'' containing the two songs, but only in the UK)
3. It's something that was available for a brief period (the 8-track release of Animals by Pink Floyd contains an exclusive guitar solo by touring guitarist Snowy White to link the two parts of "Pigs on the Wing")
4. When a band decided to pull a fast one and release two different versions of their single with different B-sides (Radiohead released two CD singles for their double A-side "High and Dry/Planet Telex" one features "Killer Cars" - one of the band's best songs of all time - as a b-side and the other features "Maquiladora" which is OK but totally not worth me paying for another import single, thus I have to download it).
5. When a band re-releases their most recent album with a brand new song or two (and call it a "deluxe edition" or something) to make fans buy the album again (several hip-hop and pop groups do this, but this has only happened to a band I like once, when British rock band Bloc Party added the non-album single "Flux" to future pressings of A Weekend in the City. This was a good thing, because at the time, the single was commercially unavailable in the US and was the only way to get it here. The bad thing was that fans needed to buy the album again to get it. At this point, an artist is just begging, "please, illegally download this song")
6. It's an unreleased rarity (several early Pink Floyd songs - like the unreleased Syd Barrett-era single "Vegetable Man" - are in this area. Members of the band have no problem with these unreleased songs floating around and are actually surprised anybody likes them in the first place)
7. The record was plain and simple never released in the United States (most of the discography of the fantastic Welsh folk band Gorky's Zygotic Mynci. In fact, most Welsh language music has a very hard time getting to the US. Usually I buy these as imports for extremely high prices (like $50. It's worth it), but on a rare occasion I can't I resort to downloading)
However, I get 99% most of my downloaded music (note: downloaded. Music that I downloaded from the internet in any way makes up only about 5% of my total music library, the rest I purchased from a store) legally off various blogs and websites (if you know where to look. Places like indie record label websites, music blogs, websites for radio stations, Pitchfork Media and sometimes the band's website have free, legal mp3s. And if you're a fan of Phish and the Grateful Dead, both of whom not only allowed, but encouraged fans to tape their concerts, you have websites like Internet Archive). Most of the time, the artist or label (usually an indie label. Majors are ruled by the iron hand of the RIAA, who will sue them for doing this) themselves mails a song (usually the first single) to blogs and websites to post so fans can get a taste of their new music so they are more likely to buy their new album (it works).
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here be dragons