It's always a compromise. You can make the MTB better for pavement, but it was still designed for off-road use. Tons of people ride MTBs on the street as they find the durability and comfort suits them, and they are not interested in optimum speed and efficiency. I like cycommute's idea of being able to go pretty well where you want to; we do that to some degree on police bike patrol.
Still, as the old saw goes, "if you want to go far and fast, get a roadster".