Now I'm not going to complain that this is earthshaking, but one of the greatest appeals of XenForo is its "Xen"-like simplicity. It has an Apple-like attention to detail in getting the basics right while keeping them simple.
In this thread, Mike explained that this was an edge case. In the battle between feature creep and UI elegance, there is something to be said for focusing developer time on getting the basics right, though.
Distant "last seen" times can make users seem less engaged and make others wonder if they are still around. Editing a post can take a while, but doesn't update "last seen." Likewise, viewing a membercard is an indication of continued engagement but also doesn't update "last seen." Because these involve communication with the server, they are instances of the user being "seen" by the server and could be recorded. If "last seen" is to be as accurate an indication of when a user was "last seen" as is feasible, it could include these cases.
Possibly I just feel bad because I gave Herbie, the guy in the picture above, a hard time for not being around for half an hour when he was on the clock. I felt pretty bad afterward. I'm taking the time to write out the post, though, because I think that details matter.