Those are different definitions. But "given that this entire thread is about efficiency", I guess the point is, when you say "serious rider", that is shorthand in this context for "rider who is serious about efficiency". I consider myself to be a serious rider according to your first, more general definition, but I am definitely not serious about efficiency. My commuter is 2x9, the gear steps are already wide because my mtb cassette is 11-36, and usually by the time I feel I need a shift, I shift two gears anyways, so practically speaking I ride a 5-speed 11-36 cassette.