I first heard about that phenomenon in this thread at the home theater shack forum. It came from a very reliable source. However, I don't have any first-hand experience.

I never encountered this phenomenon when I ran a 6.1 setup, but I also had a dipole rear surround speaker in that arrangement - which may be why I didn't run into it.

I started with 6.1, using a small Polk center for the rear. Then I switched to Polk FX300I's for the rear channels. I kept my opinion to myself and let guests who'd heard it both ways make their own judgments. Nearly everyone commented on how much more open home theater sounded in 7.1. As for music, I now always first introduce a new listener to the Brothers in Arms SACD. I get the same response. They'd say it sounds more airy and open now. I'm running the rears in dipole mode. They're sitting on short homemade stands in a bow window, nearly all the way back to the pleated shades. That's about 15" back from the inside wall and about 4 1/2 to 5 feet from the listening position. The front mains are 11 feet away.
My surrounds and rears are all sitting on temporary box stands made of 3/4" pine. I'm playing with the height, cutting them down a little at a time. They're carpeted with the same carpet as the floor. Okay, it might seem a little hokey looking. But there's no denying how it sounds. And with no missus to badger me about my grand experiment I'll keep playing with it until I get it right. Why not mount the surrounds on the wall? The drywall resonated. Sounded horrible. I want concrete walls, floors and ceilings. Hmm...a jail cell?

As for the 6.1 phenomenon, I never heard it. I liked 6.1 over 5.1 for movies. The experience of something coming from behind and going over your head to the front was very cool. If I had to choose between 5.1 or 7.1 for music it would depend on the material.

I don't believe 7.1 or 6.1 is absolutely necessary. But now that I'm there I won't go back.

A single rear speaker behind the listener can lead to back-to-front reversals, a psychoacoustic problem where sounds along the centre line can momentarily appear to come from the opposite direction. This isn't a problem with a centre speaker up front, where our human hearing is at its most acute, but it can be a problem behind us, where our hearing is not so hot.

The reason you avoided the reversal problem is exactly for the reason you mentioned: you used a dipole rear speaker. With the null pointed towards the listening area, you got very little direct sound from directly behind, instead hearing mostly reflected sound from along the back wall.

For any of you using direct radiating rear speakers, avoiding the reversal problem couldn't be simpler: just use 2 speakers, spread well (at least 30 degrees) away from the centre line. This is the main reason why Dolby and DTS and THX all recommend using 2 rear speakers, even when playing back the mono surround-back channel of their EX/ES soundtracks. Note that this is the only channel where 2 speakers are recommended for playback.