Basically what Liontamer, seanne, and Akumu all said
What got me into doujins was the chance to hear stuff that would never get arranged/remixed any other way (I started with salvation by faith records releases). What kept me listening is the quality (most of the time) and the price. Why bother buy 3-4 official releases of mediocre quality when for the same price I can buy 10-15 bite-sized arrangements that usually kick the pants off anything the 'biz' is doing?
Ex. That terrible Techno Rockman arrange vs. Rock Love. Love hands down.