If a site seems to 'work' - the visitors (mostly new) appear happy with the content/structure etc, and the search engines are happy to rank it - is there any value in making small, regular changes to the content to make it appear 'fresher' to the engines, even if this does nothing for the visitor ?

I've read that fairly static sites may get crawled less frequently but so what ?

I tend to the view that 'if it ain't broke don't fix it' - unless you can make the visitor experience more worthwhile. On the other hand, algorithms don't have feelings.