I think I have been particularly bamboozled by having passed through
Wimbledon station lately. There _all_ Oyster users arriving on Tramlink
are
warned to validate at their originating tramstop in order to be able to
exit
at Wimbledon (though this advice no longer appears on 'Ask Oyster', just
the
website) and I've never figured out why. I can understand the need for an
entry to the system to pair up with an exit but I would have thought it
would be possible for National Rail users to also have arrived without
having validated at their origin.

OK thinking about it more I realise I was only thinking in terms of the
honest punters and I can see this being useful as an anti-fraud measure to
prevent anyone travelling on the tube/national rail without the appropriate
ticket using a relatively cheap tramlink pass to allow them to exit the
station.