If you are into multichannel music, like playing DTS-CD, SACD and DVD-A, direct firing surrounds that are IDENTICAL to your fronts are ideal. That's because many multichannel mixes have full range dynamics on each of the 5 channels. OTH, if you're more into movies, either dipoles or bipoles are recommended because they provided a non-localizable dispersion trait that can be best serve ambiant surround signals.
Having said that, some HT and music enthusiasts have both. They can select which depending on the source material they are playing via a dedicated rear amplifier with speaker A/B switching. There are also multichannel music mixes with ambient concert hall information in the rears that can better be served with dipoles of bipoles. These are mostly jazz and classical mixes. So having the flexibility to select between the twp types would afford the most benefit depending on the source materials. But one doesn't have to have dedicated rear amps for this. A speaker selector box will do.
In my case, since I am more into music and multichannel music, I have identical rear and front floorstander setup. So far they seem to do as well in movies. I would like to go the selectible route too, but I just don't have wall space for dipole sets.
