I battled the thought of adding a rear to my 5.2 setup... because I have my system laid out in the corner, with my Plasma in the corner, then my rear would go in a corner as well. I honestly thought it wouldn't make a big difference. I bought a single Di-Pole for that rear corner. I thought my system sounded amazing with my 5.2 setup, but adding that rear made movie watching a completely immersive experience setting with the right movies. It sounded amazing, and I wouldn't go back to 5.2.
The choice of rear speakers (Direct or Di-Pole) IMO depends on how large a room you have, and how far back from behind the listening position you can get. And your intentions of use, Movies/Games/Music.
Also keep in mind, if you do alot of gaming, the 7.1 soundfield is quite accurate. Things coming up from behind should be localizable and are best reproduced with direct radiating speakers, Di-Poles don't reproduce this very well at all. I have found they tend to just smear those sounds out behind you... I don't do any gaming, but I have had some experience with it on my setup for a short time.
For movies I found the rear Di-Pole works in my environment just fine, and movies are a completely different, 100% better. Many fanatics will argue about what speakers should be placed as side surrounds and rear surrounds.
To me it really comes down to your room layout, and choices of material.
Music will not sound right coming out of a Di-Pole speaker, it can be done, but with poor results. Some rear speakers are switchable from Di-Pole to Bi-Pole.
Obviously results will very, as well as opinions...
Warp