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.