Have seen laborers and blue collar workers, my father included, that have to use their knuckles because their finger tips are too callused and dry for touchsceens. Seems like many groups have these kind of issues.
When I got to college, they gave everyone Thinkpads with fingerprint readers. Mine had trouble reading my fingerprints for about 2 months. I guess that I scrubbed my fingerprints away with all that grease, and it took that long for my fingerprints to "regrow" enough for the sensor to recognize them.