Because if you didn't know about it before and you thought it was crappy and Microsoft, there's no helping you, Mr Preconceived Notions?
It's only just gone multi-plaform now, but it's always been open source (what with the slightly dubious legal status of compiled versions), with a wealth of codecs and stuff. Given the extreme lack of alternatives, Softmodded Xbox + big HD + XBMC = the best way to get a large-capacity media jukebox on the cheap!