Paul Scott wrote:

Or just enforce that touching in and out
is mandatory regardless of what's on your card.

Then explain how you'd handle the following situations:

1). I sometimes visit my parents in Epsom, for which I get an extension
to my existing season ticket. I will not be able to touch out at Epsom so
how do I touch out. (And even if I can get special dispensation to enter
through the barriers without touching in, I may well not be on an Epsom
train for all of the journey, but instead changing at Sutton.)

Bad example Epsom is irrelevant to the 'OEP discussion' because it is
outside the zones. You'll have to do exactly what you do already - ie buy
a ticket?

I buy an extension (and *really* hate the queues to do so) but still have to
touch in when entering at the London end. If it was compulsory to *always*
touch in I'd end up with penalties for not touching out when it's not
possible to do so at my extended destination.