The only thing i don't like about the 36" stands is that some couches have high backs. If you were to use them for a rear centre stand, the speaker could potentially sit below the listener ear behind the couch/seat, which i think is one position that will not work even though as fhw says, the quadpole dispersion is quite forgiving.
This is not to say that sometime in the future we won't have a 15.1 system with speakers under the couch, in the couch, above the couch, beside the couch, behind the couch....
