Who Uses Rear Surround Speakers For Music Playback?

There is a rational for it. You can hear the "size" of a venue by how long it takes the echo to reach your ears. If you have the ability to delay the rear speakers you could mimic the size of any venue by adjusting the delay and relative volume. You would not need much of a loudspeaker either. It would not be an inexpensive thing to do. You need two speakers, a two channel amp and a digital processor to manage the delays. $5K anyway to do it right. Maybe some day when I have a bunch of spare cash lying around. 

For me it's a NO. But whatever sounds good to you.
Fuzztone - Be more polite and no racist comments.

I think it really has to do with the source material. Some music sounds great up mixed to surround. If you can adjust the center fill on your pre/amp or receiver in Dolby Music setting you can really get some good results. One of my favorite albums to listen to in this mode is Radiohead's Kid A.

I use the rear speakers and center when I listen to classical multi channel music. Some that are recorded well are fabulous. Some not so much. It depends on whether the recording was actually made that way.