Wears out the throw out bearing, disc and pressure plate prematurely. Believe it or not, you might think you do not have a lot of pressure on it but if it moves a little it is putting pressure on it.
BTW, during a ride along with my old boss, if you left your foot on the clutch you did not get hired.
2013 Performance Package V6
2013 F150 6.2L Boss
Yes it is a Boss