Basically we're talking about the difference between professional entertainers and yer basic folksingers. If you're getting paid (and hopefully well) you will not make your audience sit there in uncomfortable silence while you tune or regroup... you have stuff to say. Hopefully it's something you've rehearsed so if the muse doesn't hit you, you have something clever to say that doesn't sound "read". Given enough gigs you come up with stuff on the cuff, but sometimes the old line is just too too perfect, and the point is to show THEM a good time...musically speaking of course!