The short answer: Why bother? The shorter answer: Why not?

Toyota says the Prius is neat because you don't have to plug it init charges itself when running on gas. But what if you could plug it in? If you could fully charge a set of supplementary batteries overnight from household current and do the next day's errands purely on volts? (The gas engine will stay off under 35 mph if you have enough battery life and you're gentle on the throttle.)