I suppose the argument goes: if anyone can approach that optimality, it's a company with a large fleet of repurposable drivers.

Being able to dispatch to a driver in motion depends on having a suitable driver available.

I'm still doubtful it can ever work, for the reasons people mentioned above. But the realistic game is that Uber becomes the new Yelp and strongarms restaurants for a share of their profit, in return for customer access / promotion. (Which I have pretty strong feelings about as an ethical business model, but that's just my opinion...)