dmorison - 11:23 am on May 22, 2013 (gmt 0) [edited by: dmorison at 11:27 am (utc) on May 22, 2013]
I think LastMod is certainly worth using if you are able to provide it accurately and dynamically (so less likely to be the case if using a generator). One of my sites has a relatively large forum, which is mapped with LastMod set to the date of the last comment on the thread, which works really well.
An intelligent search engine, assuming that it has gained trust in my sitemap can then allocate resources more towards indexing new content and my main pages more frequently than it otherwise might do so.
I don't use Frequency (redundant anyway if you support LastMod) or Priority at all, but I think the intention is just to serve as hints to search engines, which are pretty good at working that kind of thing out for themselves!
[edited by: dmorison at 11:27 am (utc) on May 22, 2013]