Forum Moderators: phranque
If you are updating regularly, then your fine, but like TMI says, if you don't update, it becomes downright embarrassing.
I frequently place a last modified on pages for two reasons.
1. To let the viewers see that the page has been updated recently and that the information is current.
2. To let the viewers know that the information is old and, perhaps, out of date.
For any "last-modified on" for this second purpose I wouldn't recommend that a scripted date inclusion method be used, since it's a little too easy to accidentally "update" your purposefully "old" pages this way. Instead I typically hard code the date in.
Using a "last modified on" in your pages certainly doesn't carry any weight in search engines (That is unless you want to use the date as a keyphrase). However, a search on the Google forum here will turn up a few threads on the usefulness of having your server correctly send a Last-Modified header, which can lead to more efficient Google spidering.