In order of precise imaging and localization of sound source (greatest to least):
1. direct-radiating (monopole)
2. bipole
3. dipole
Conversely, in order of perceived size of sound-stage (largest to smallest)
1. dipole
2. bipole
3. direct-radiator
Because dipoles have the two sets of speakers out-of-phase, you actually get a null on both sides of the speakers. This is why they were the preferred type of surround speaker in the old Pro-Logic (version I) days, where there was just a single surround channel, and the objective was to have that channel appear to be both around and behind you, with as little localization of sound as possible. Dipoles also have limited bass output, due to cancellation of the out-of-phase responses of the woofers or mid-bass speakers. Again, this wasn't a problem with pro-Logic set-ups, since the surround channel was band-limited anyway.
With 5.1 (and greater) systems, with 2 or more separate, potentially full-range surround channels, it is more important to have at least some level of bass response, and (arguably) better imaging and sound-localization. Bipoles therefore represent a good option for the surround speakers in 5.1 (plus) systems.
If you are using a sub for all of the bass content (below your receiver's crossover freq.) anyway, then a lot of the distinctions between dipoles and bipoles diminish.
As to bipoles versus direct radiators - depends on your personal preferences. A fair number of people (most) like bipoles (or dipoles) for surrounds, but not for L/C/R speakers. But there are also a fair number of fans for bipoles at all speaker positions.