The larger the gap between the gears, the less "smooth" the shift between them will be.

Look at the cassette (The rear gear set) on the high end bikes, and you'll notice that it's a very tight group. This isn't JUST for fine tuning (although it is nice to be able to fine tune your effort like that) but also to promote smoother shifts.

That being said, there is a small but growing movement towards internal hubs in back (which shift well over large gaps) and a single chainring up front.

-- James