This forum post claims that some future iTunes songs may be published without one or more tracks to allow for playing along by the listener
Today I put together different mixes from the new band Damage Plan (formerly Pantera) to be sold on iTunes. The neat thing is each track has the song without certain instruments. Track one is without vocals, track 2 is without drums, track 3 is without guitars, etc... They did this so their young aspiring rock star fans can add their own instruments and play along with the tracks.