Mark, I see your point, and yes, demo players should incorporate some of these techniques into the demo ...
Obviously, some of the things you mention - such as changing styles in the middle of a song - are dependent upon the venue and the audience ... I would not change a Latin style to a Swing style if people are dancing, but I do it when playing "Green Dolphin Street" in a restaurant setting, or playing Louis Prima's "Buona Sera", or other songs that lend themselves to it ...
And as for using some of the other variables such as Intros and Endings within a song, because I'm doing it, I would think other OMB performers are also ...
But I guess there are those who just turn the machine on, select a style, and sing or play over it ... They had better be a great vocalist or player, otherwise I just don't know how much work they get ...