> Is it really that useful to identify someone who has overstayed their visa when they are leaving the country? Sure, you then know not to let them in next time.

This happened to me leaving London and it was a lot of fuss over nothing. I (American) had given my forwarding travel plans to the border agent upon arrival and I didn't notice but she marked my passport with a visa for an exact stay (just the weekend). Except she mistakenly wrote my exit date a day early. It took quite a bit of explaining to convince them not to put a giant "DEPORTED" stamp in my passport and even though they eventually believed me that it was a clerical mistake they definitely kept track in their system because I was questioned about it several months later when I went to the Falkland Islands.