I think the screensaver thing makes sense. I know that an image can "burn in" to our tv if left on for too long. My Bernina does not have one of those computerized screens, so I didn't think of that. Don't really know if it's a problem with screens on sewing machines, but might be worth checking out.
One thing to keep in mind if you are turning the sewing machine on and off frequently is that you might be wearing out the on/off switch! To my mind, that's a much bigger repair than simply replacing a burned out light bulb.