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.
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Yes at least google loves fresh content.

There are two very good reasons to keep your site content fresh:
1) Search engines do like fresh content, and sites will go "stale" and rankings will go down over time if they are not updated at least occasionally.
2) Your competition will not stand still -- they will be working to get above you, and if you don't keep optimizing and link building, you will drop relative to them.

You don't need to change content that addresses your core value proposition or call to action if that is working well -- change things around it. Experiment with content, keywords, etc that attract new keywords, tail terms, etc. Add more content to your site on other pages that are relevant to your value proposition. Add a blog or wiki to get users involved and attract back links.

Bottom line is you do need to keep your site "alive" to compete in the long term.

yes so you have to update lots of fresh contents!