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.

I think you've got the right idea.
The search engines won't keep crawling your site as often if there's nothing new to index, but your pages will continue to rank about the same until something better (not just "newer") appears. Or until they re-jig the algo! I have one home page that has been bolted to the top of its main SERP for 4 years - even though I've never changed my story on it and there are many new competitive sites.
Changes to improve customer experience make sense - but I figure that's the only reason to change!

I agree with the common "dont change it if its not broken" however if you are trying to build a site like daniweb then you content has to stay fresh. To get regular viewers you need to offer them new materials to look at or to help them or else they may end up going to a new site for their information or material.

Ask yourself this:

Who am I trying to make happy, my visitors or a crawler? Code for your visitors first, then worry about the SEs.

That's what I do.