OK, I'll give it a go.
A dipole has two sets of drivers on opposite faces/sides of the speaker, driven 180 degrees out of phase. By this definition, most "planar" speakers are effectively dipoles. (One example would be Magneplanar speakers, where a thin film or sheet is vibrated in a field to create sound. Since there is just one film vibrating, the sound wave off the front of the film, and the sound wave off of the backside of the film, are identical, but 180 degrees out of phase.)
In a more conventional speaker design, using dome tweeters and conical drivers for mid-bass frequencies, you would just have a two matched sets of drivers on both the front and rear faces of the speaker, wired in opposite polarity, so that the signals from front and rear are 180 degrees out of phase.
The effect of a dipole speaker is to get sort of a figure-eight shaped sound field. Sound is basically sent in all directions, except there is a null, or cancellation area on both sides of the speaker, perpendicular to the two sides with drivers. Since bass response is less directional than treble response, these nulls or cancellation areas tend to translate into reduced bass response.
Back in the days of basic Pro-Logic, with just a single rear channel signal (albeit typically fed to two rear speakers), the goal was to have this rear channel dispersed as much as possible, for an enveloping effect. By using dipole speakers, oriented with one of the null areas pointed at the listener (i.e., with the drivers aimed perpendicular to the listener), the listener was basically hearing reflected sound, with little directly-radiated energy.
Now that we have discrete 5.1 source material, there is more reason to have greater directionality and imaging precision from the surround speakers, and so there is more use of non-dipole (either direct-radiators or bipoles) surround speakers, and/or orientations that aim one set of drivers more directly at the listener.
As a side comment, bipoles are basically similar to dipoles in having both front and rear-facing drivers, but the difference is that the two sets of drivers are wired in phase. This gives more of a 360 degree sound dispersion, without the side nulls or cancellation areas. When bipoles are used, it is more typical to have one face of the speaker directly aimed at the listener. Although, in the case of a bipole used as a surround speaker, the orientation can still be changed (as with a dipole) to get either better imaging, or more dispersion of sound (with the drivers aimed perpendicular to the listener). As an aside, some dedicated surround speakers have the two sets of driver faces at an angle to each other, rather than totally on opposite sides. These are sort of "pseudo-dipoles", or "pseudo-bipoles", as the case may be. Since it is pretty easy to build some polarity-reversing switches into the speaker design, some surround speakers are switchable between dipole mode and bipole mode.
As noted above, bipoles will tend to have more bass output than dipoles that use the same drivers. Bass response for the surround speakers didn't really matter much in the old Pro-Logic days, as the bass response of the surround channel was limited. But with discrete 5.1 there is at least some advantage to having improved bass response for the surround speakers.
