In what devices Nigel? You obviously have specific devices you have in mind when you think of Mosfets in a switching power application which obviously don't speak for all power mosfets in a power switching situation. Mosfets are used extensively in switched mode power supplies for motors and for UPS supplies. You seem to be talking TV sets when that is not the only place where such devices are used. In the case of a piezoelectric driver where distortion isn't even considered in general as any harmonics created can't propagate because of attenuation mosfet drivers can be extremely efficient.