most go with a dipole because of the compromise they may have in their room running a monopole surround too close to their listening position... if the speaker isn't very good at imaging, or if the location is so compromised (layout issues, distance to the listener's ear, WAF placement issues, obstructions, etc.), the dipole tends to restore the sense of spaciousness to the surround channels, and is much more difficult to place the speaker. It immerses you more in the surround field instead of hearing distinct speakers and ruining the illusion.
Of course, if the dipole speaker stinks, the the whole point is mute. The TS-150 speakers from Von Schweikert (now discontinued) were really good dipoles for the money. I was startled a couple times in installations where you should have been easily able to tell the speaker was right there how much they disappeared and allowed the illusion to take over of enveloping soundfield.