If the contrast control were software based this would create some forseeable problems. If the contrast setting was turned down extremely low, then some people would complain that once the setting was turned down that low they could no longer see the screen to readjust the contrast back up! A response of "what about adjusting the contrast of everything else on the screen except the contrast control?" is an impossibile request. The nature of LCD hardware and the way in which the contrast is controlled makes creating this type of interface nearly impossible or ridiculously costly at best.
A wheel provides the quickest form of contrast adjustment and greatest degree of sensitivity. I have used hard button contrast controls and I have two complaints about them:
1. In order for you to get a feel for how much you can increase/decrease the contrast from the current setting, a bar of some sort is required. Even the fastest responding contrast controls of this type are, in my opinion, too slow for a handheld display device. Most PalmPilot and Windows CE handheld devices use wheels for this very reason.
2. Wheels give a greater "feeling" of contrast sensitivity as opposed to hard buttons. Some people would undoubtedly complain that the various levels offered by button control are not in small enough increments.
