I was browsing Masterbeat and noticed that all navigation is in-page, using either Flash or Ajax. There is only one page for the entire site.
The reason as always with music sites is to allow the user to browse without interrupting playback. Not interrupting the music is such a pressing need that I can imagine virtually all music-oriented sites moving to this development paradigm.
Other music sites doing it: Hype Machine, Bandzoogle, Critical Metrics.