blinky,

Any speakers you attach to Main B would only be carrying the same front channels that Main A do. If you also listen to music, these can come in handy because the "ideal" :) location for the main front speakers is not the same for music as for home theater. So, you could set up "A" pair at the best locations for theater and "B" pair at the best for music... I would not use both pairs at the same time, for the reasons outlined by tweakman.

-Bruce