The main advantage of proper bike shoes is power transfer to the pedal. The sole of the shoe is rigid, allowing for maximum power transfer. Shoes for running or other activities will have flexible soles, which will lead to power loss, and thus slower possible times on the bike. I can only imagine that the sole of the fivefinger is even less rigid.
Yes, the fivefinger sole is completely flexible.
I've always wondered about this 'main advantage of proper bike shoes', though. The same theory applies to running shoes, thus the arch support and generally rigid form. There's some evidence it's not accurate and that the arch of the foot transfers force wonderfully. That, in fact, is part of the reasoning behind FiveFingers. I'm not trying to be controversial, and I don't use FiveFingers on the bike currently, but I've always been suspicious that the rigid sole isn't magically better. I know clipless offers other advantages too of course, just musing about this bike-shop piece of wisdom. (edit) I've used fivefingers for about 400 miles, thus my thoughts. On the other hand, I don't do long distances at the moment, or bike races, which may completely disqualify my observations!