I wouldn't mind the use of an aid, (after a certain age people use aids and pills in order to please...
) but dmg's point is solid, though.
I believe that I wouldn't really mind the occasional slip of the tongue or blank mind, just the feel as twm states for Dylan. Of course, for me, Bob is the exception of the rule, since I really try very hard to figure out what he is singing. So i don't see the need for lyrics for Dylan. Perhaps he is just looking for what joke to say.
Seriously now, the key phrase is "pay to see them play the guitar and sing". If we were casual concert goers, we would decide to go to a concert partially based on the ticket price and we would expect a certain level of performance. So, based upon our standards and expectations from other artists, we might be very disappointed or extremely thrilled from a performance that is just average for the artist. On the other hand, since we are not the ordinary fans, we can be both thrilled for seeing the artist live and at the same time objective with the performance, because we have plenty to compare with.( Glad to see him but today's concert was really crappy. Remember Royal Albert Hall in 1996? That was really something!
)
Or maybe it is the other way around?