The question made me think and here's the main reason I came up with. Buying an item emotionally invests you in it. You'll look after, cherish and protect it. Of course I should also point out the major selling that has stopped me from purchasing music online up to now has been DRM. iTunes Plus will do for now but I'm really waiting for the Amazon MP3 store to come to Canada.
There are two main drawbacks to me for buying music. One, I'm completely out of the loop when it comes to "what the kids are listing too" and two, my disposable income can be better spent elsewhere.
So here's my solution, download legally free music. You might have to do some leg work to get them but no more than you would do running Limewire or Bittorrent.
Believe it or not my knee jerk first choice here is iTunes. I've mentioned this to people and very few are even aware that iTunes has a free single of the week. Ok, it's DRMed, and only works on an iPod, but it's free and if you don't like it delete. That will get you 52 free songs a year. In the past year I even scored a free audio book!
Your second choice is Puretracks. They have a free MP3 of the week also but unlike iTunes it's an unDRMed MP3 encoded at 192kbps versus a 128kbps AAC file. That's another 52 free songs a year for you.
The mother load for free music is download.com. They have a free song a day, no DRM and encoded at 192kbps. There are over 111,000 free songs on their site and you don't have to be a member or be located in the USA for get them. A plus for me. So there's an additional 111,000 songs for you (coincidentally, that's 37 times more songs than my current music collection).
Of course this is just scratching the surface when it comes to free legitimate music. Many independent artists and labels have free songs. Armed with time and Google I'm sure you could certainly find more, who knows you might already know of some. And if you do why not let me know. Thanks. I can't think of a better serendipitous way to discover new music.
With so much free music out there it not only makes me wonder why people download music illegally but why anyone pays for it either.
UPDATE: Dan of we love music passed along another site that offers free downloads.