When I play with the bluegrass folks in Seattle, most of them have "The Fiddler's Fake Book." If they don't already know a tune, many of these fine instrumental players are, understandably, reluctant to do a melodic break without "the dots." Just watching someone do the guitar chords won't give you the exact tune.

Once folks know the tune, they, of course, don't need the dots. And you can learn a tune by hearing it repeatedly. (Another way of saying it is that one can scan a page of music visually many times in a minute, but you cannot hear a 2-minute tune repeatedly in one minute.) But with a simple score in front of them, many instrumentalists can PLAY the tune BEFORE they memorize it, and that playing is a good way of learning it.

Many times I'll be at some kind of session where someone wants to play/sing a song that someone did, say, the previous week. If they rely on memory or a lyric-chord sheet, usually they can't play and sing it without the presence of the person who introduced it. But if the tune is printed in simple "dot" form, there's no problem for the ones who do read music.