Al these uses are not very well served by the 'changed:' field right now. People can change them at will (fun to confuse people), the timestamp is not UTC (very annoying for people living in other timezones) and it doesn't provide the time of changes (interesting if you make more then one change a day).
I think we look at this field in a very different way. For all I care it could be my own internal person-id and date that is indecipherable to anyone else. It doesn't matter to me about UTC either because my own time zone doesn't change... perhaps this would matter to an international backbone provider with NOC's in multiple time zones... so I guess I can see that point, but only if you happen to be such an entity. Why even define it to be an email address? Why not just make it change: company-internal-id company-internal-date-fmt