The point of 5.1 is to playback 5.1 sources. Without 5.1 sources, there is no need for back speakers.
When you have 2.1 speakers, the channels are usually mixed together. If you have 5.1 sources, 5.1 sounds better, but if you don't, it doesn't.
So, most DVDs will sound better with a 5.1 set then.
Ugh. I don't know what to do.