Generally for diagnostic purposes or the " he can you tell me what that noise is" thing, but riden many bikes not my own, and have no real problem allowing other to "have a go" on mine as long as I've confidence in their abilities.
But then being able to fix just about anything helps I suppose
Memorable moments, Dan pulling away from Soho on my Speedtriple (and the look on his face when I had to go and re-start it
), and Swiss once I got back from testing his bike at an early Selkent, the line was " I didn't know it could sound like that".
Cheers Mark