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.

No. It's all about content. If your content is interesting and generates inbound links, then you are doing what you need to do. To keep it interesting, you need to keep adding content. A site is always a work in progress. New fresh and interesting content will draw traffic and inbound links and that brings the search engine back to your site more often.

Don't design for the search engines, design for your visitors.

get a site map on Google Webmaster Central and keep adding new pages... you need to get others to see your site and link to it.... forums, blogs, article submission sites and becoming a writer for info sites can help build your name and links