Depends on what your listening percentage (HT/Music) is, and what type of sound you prefer. Bi/dipoles will give better diffused sound, while direct radiators will give more localized sound.
I cannot install dipoles in my particular room setup, with an open wall on one side. Personally, I prefer direct radiators anyway.
I have a mismash of speakers in my system now, but in a couple of days I am getting my Axioms. Their surround speakers are Quadpole (woofers fire up/down and tweeters fire out on 45 degree angles).
If you have only a small listening area then directionals should be fine, whereas the greater the listening area the less people will hear the full surround effect.
Dipoles/Quadpoles will disperse the sound around more, thus giving a larger listening area. (which I need since I have 2 rows of 3 seating)
I feel that more dispersion is what is intended on the surround channels(at least that's what the THX guys recommend)
Yes for multi channel music the monopoles are must,for movies bi/dipoles are preffered,by many,however for soundtracks that are music "heavy" the monopoles are better IMO.
IMO, I prefer direct radiators or bipoles for both music and movies. If you wish to wall mount your surrounds and will use your system primarily for movies, then dipoles will work also. I have surround speakers that can be configured as either direct radiators or bipole, and I found that in direct mode they are too point locatable for most movie playback. IMO, surrounds are not meant to be focal points which draw out attention away from the movie. For 5 channel discrete music such as DVD-A and SACD playback, I prefer the speakers which match my front channels... in my case, bipolar. I do however, use dipoles for my rear center channels since I WANT difuse center rear sound and my room layout demands they be placed very close to the listener's head.