I was wondering if refreshing the lastmod may have an impact on the frequency of crawl by Google? For example: if a sitemap is lastmod on 2nd Feb, and Googlebot visits it on 3rd march, it may not crawl as much as if it was lastmod on 2nd March? Any idea? And how can you automatically change the lastmod in say, 200,000 links in your four sitemaps, everyday?
lastmod should only be used when you have the pages on your website which changes frequently. Suppose you use a lastmod for a page that you want to change weekly but you don't. Now when the robot will come to your website, it will see that this page was to be changed weekly but it is not. So, it will not follow the lastmod and will crawl the page as per its own will. The best thing to do in XML sitemap is t use loc and priority only. Use lastmod only when you are going to change the content frequently.
But that is not what I asked. I do realize that I should not place lastmod more than I change the content. In fact, I am asking whehter keeping the lastmod date fresh (if your contents are changing daily) makes any difference?