If you using your iPad in a harsh environment where there is heavy dust, dirt, sand etc then maybe some protection for the screen is a good idea. Most people using their iPad in a controlled environment like their home or office are pretty careful in understanding that they are working with a piece of glass.
For most, having something between the screen and their fingertip to "protect" the glass is overkill. Apple has engineered the glass to be pretty durable under normal use. So again, if you're in the desert or other tough environment, maybe it's necessary, otherwise like williantaylor mentioned above, a good cover or case should be enough.