Maybe I'm missing something obvious, but I can't think of a reason for scrollbars to exist in the first place. You always have a physical way to scroll. If a user doesn't know how to scroll, it's a problem with the device, not your software. The only purpose of the scroll bar is to show your current position in the page, which doesn't require persistence.
It's sometimes also the only indication that an element is scrollable. Usually there are other cues, an unfinished sentence or partially clipped text/graphics, but sometimes you're left with a perfectly plausible looking clip and would never guess there's more underneath.
