Well I listen to a variety of genres of music and I buy movie soundtracks, anime and video game soundtracks, heavy metal, hard rock, and japanese pop/rock etc. As I mentioned in the first post, some of the labels, especially those releasing older movie soundtracks usually limited edition stuff tend to give plenty of samples that I could easily determine if it was for me by sampling what is provided.
However, the labels that put out soundtracks from new movies and TV rarely have many samples posted. Varese Sarabande used to post four or five decent length samples on their web site from a new release but has stopped posting samples altogether. Anything I buy of Japanese origin I will download first just because of the cost of importing. Most Japanese bands frequently release singles in anticipation of a new album but they aren't always represented on the album.
Then finally there is heavy metal [I use loosely to include all types of metal I enjoy] in which most labels will provide one track to sample if you are lucky and a few bands who are more independent might post three tracks on their myspace page. I have been lucky that a few of the recent albums I bought based upon the one track available turned out to be good.
In my experience bands who are independent enough that they post a ton of music on myspace and youtube I probably wouldn't know were releasing music unless I stumbled upon them somewhere. Those types of artists would benefit from illegal downloading because the user who sees a familiar poster putting up a debut album has the benefit of their past with the uploader and might check it out when they might otherwise ignore it.