Mot many, but it's hardly ever the case that people don't have more than one application open at a time. Your mail program could be doing stuff while you're browsing.
Since web browsers are very often used, it's worth mentioning that most make use of multiple processors by running multiple threads and at least Safari (not sure about Firefox) runs the plugins as separate processes so that Flash, for instance, which is a complete CPU hog, will consume one CPU but not the other which would be free for Safari.
That said, I highly recommend ClickToFlash for all computers because Flash is such a hog. It's much more useful on older, slower systems.
OS 9 multiprocessor use is extremely specific to certain applications, so yes, a dual CPU won't give you any benefit at all unless you're running one of those certain applications.