If emails flow in or out of a ticket, the Last Update field does not reflect the activity. Specifically, these emails would be between Techs, not to/from the Requester (if that makes any difference). I saw another forum post that would seem to indicate that emails involving the Requester would in fact update Last Update field, but I can't speak to that at the moment.
Emails touching a ticket, regardless of who they are from/to should, in my opinion, be reflected by Last Update field.
Can the intended behavior be clarified, and if it is working as designed, can a comment be left whether this may be changed in a new version, and if so, whether that would likely be soon, or a long ways away?