Scrolling is one of the most common operations in most user interfaces, so it's not surprising that we have special hardware for it. Scrollwheels (and trackballs, the two-dimensional equivalent) are now common on mice, and on some other devices such as Blackberries. But this is a fairly recent phenomenon. Fifteen years ago, almost nobody had hardware for scrolling.
This is significant because it affects user interface design. We're accustomed to having dedicated pointing devices, and consequently our designs assume that pointing is easy. (And when this assumption is wrong, as it usually is on laptops, our interfaces can become quite clunky.) But we're still getting used to having dedicated scrolling hardware, so we design as though we didn't. We waste screen space on scrollbars, even though users rarely use them. (I only use mine when my scroll ball is gummed up.) We try to avoid having to scroll, because we think it's cumbersome. (And it still is, even with hardware, but it's not quite as bad as we're accustomed to.) We eschew spatial navigation in favor of more abstract interfaces, because we don't expect to have a good way to express movement in space.
Standard scrollwheels do have some annoying problems: their quantum is large, so it's impossible to scroll slowly or smoothly with them. This could be overcome with small changes to the hardware, or entirely in software by smoothing the scrolling. And they require repetitive movements to scroll long distances, which may be an RSI risk. Maybe something like a joystick would be better.
Any interface designer knows to optimize the most common operations, and we should remember that one way to do this is to use what the hardware gives us. If scrolling can be expressed only clumsily, via pointing, we should minimize it. Now that we have hardware for it, shouldn't we exploit it?