Here's another angle: there was once a requirement to taxi a machine from one parking-position to another due to heavy influx of traffic on the day. My colleague was due to crew-change if I remember correctly, and the question was posed whether I, rated and current PIC on the thing, was legal to taxi it by myself, without a pair of eyes in the right seat to keep an eye on that other wing while manouvering around on a slippery and busy apron, it being certified as a two-crew ship, etc.? Legal if marshals outside? Interesting one, I thought.