The dual bananas, because they are connected together, actually exert force on each other, so to speak, making the connection much more sound. The 2 banana contacts have 2 holes to guide them and hold them. This puts less stress on the metal contactors. I have had problems with single bananas loosening up, right in the connector, because the banana metal contactor seems to have less grip as it ages, due to deformation of the "leaf springs" of the connector. This could be due to larger speaker wire resulting in strain on the contactor, and I think the design of the connector itself looses grip in time. I have had some connectors actually fail having no spring left in them at all. This resulted in very poor (if any) contact. Going with duals solved that problem forever.
Wayner