When I first set up my 5.1 system, I started with a couple of small direct radiating wall mounted rear surrounds and found them lacking -- just not enough "horsepower" when the program material called for real activity in the surrounds and woefully out of balance with the rest of the system. Since I already had Def Tech BP-2000s right and left front and C/L/S 2000 center, I went with Def Tech BP-20 towers in the rear, since, at the time, they were the equivalent of the BP-2000s without the "internal powered subwoofers" (which I ended up disabling anyway in favor of a stand-alone Velodyne FSR-18 subwoofer for smoother, deeper, more authoritative and musical bass.)
I have found the BP-20s to be excellent performers in their surround role. After trying a number of different locations in the room, I have them positioned sideways along the back wall, each about a third of the distance across, which yields a very widely dispersed, seamless surround field. (The BP-20 was replaced in the Def Tech line with the BP-30. (If I were starting over with another Def Tech system, I'd put BP-30s all around with a separate subwoofer.)
Bottom line, bipolar tower surrounds work just fine despite what THX or Dolby might recommend.
Good luck!
Burke