Many of the groups I listen to do do this -- this is certainly not that rare. Sometimes they go for a bit more money by releasing a separate CD with karaoke tracks, for example, but at least if you want it, it's available.
Of course, you can very easily get just the vocal track by subtracting the two. Sometimes the "non-vocal" track will still include backing vocals or the like in appropriate places, and just pull out the main vocal track.
For music where the vocal tracks aren't released separately, you can often pull them out nevertheless. The best is if you can get the audio in 5.1 -- vocals are almost always center-panned, which makes extracting them quite easy.
Of course, you can very easily get just the vocal track by subtracting the two. Sometimes the "non-vocal" track will still include backing vocals or the like in appropriate places, and just pull out the main vocal track.
Some musicians have even released every single track of their work separately; see "Desperate Religion" on http://en.wikipedia.org/wiki/Trilogy_(ATB_album) , intentionally inviting remixes.
For music where the vocal tracks aren't released separately, you can often pull them out nevertheless. The best is if you can get the audio in 5.1 -- vocals are almost always center-panned, which makes extracting them quite easy.