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!

There may be a value, for the time when you make a change that you actually want to be crawled as fast as possible. For me, the time it would take to manually do such changes wouldn't make it worthwhile, although I guess you could automate it completely. The benefit is still rather minute IMO/for my needs.

Yeah for crawling purpose you can definitely change a little bit of the text here and there without changing the meaning of it (obviously) if there is nothing new to add and that content will appear to be new for the SE’s. It’s basically just paying with the text and it’ll be helpful to the site

Can you elaborate how often and how much should be changed on the site to get a bot visit it.

You can add meta code to the top of your pages, to get the search engine to crawl the site and pages daily, regardless if you've updated them or not. This is something I feel is more worthwhile, since you know that when there is actually an update, it will get reflected as quickly as possible in your search results.

AFAIR there is a way to tell the SE not to crawl the site more often than <insert time period here>, but not a way to dictate a search engine how often to crawl it. At the end of the day, that's entirely to their discretion.

All this can be accomplished using the robots.txt file: http://en.wikipedia.org/wiki/Robots.txt

Well the majority of search engines do automated crawling. Google has always followed the meta expire tag for us, and has revisited daily, when we have used:

<META HTTP-EQUIV="EXPIRES"
CONTENT="Mon, 30 Mar 1976 11:12:01 GMT">

Using a date in the past tells the search spider that the document has expired, and to schedule another visit on the next day.

I should add however that some engines will simply remove expired pages from their index.

The best way to keep this from happening is to generate your expire meta tag with a server-side language that updates the date to tomorrow's date every day, or with javascript that does the same. Server-side scripting is the best bet however, as it creates a static source for the spider to read.

Interesting about the META expires content. That shouldn't be too hard to do in PHP, thanks =]

It's not. You simply use it's header() equiv.

Fresh content means quick visits by bots. So search engine really like fresh contents and get bored to visit the same text again and again. So better to make few changes so search engines feel better to visit your site.

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.