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.

I've only just got mine (Z1/2) so I'm learning too, but I understand they
would like everyone to touch in and touch out. At the moment, the system
"trusts" travelcard users to not stray outside their zones (and why not, if
you're caught out of zone you'll be PF'd as you would be had you got a paper
one). It "distrusts" pre-pay users and stings them for a full all zones
journey if they don't touch out, and/or leaves an unresolved journey on the
card.

In that respect, why should the Oyster be any different to a paper one? If
you have a two zone pass and you join the tube at an ungated station why
should you touch in? My route takes me into the system at KX anyhow so I
always touch in/out, though someone at work with a Z1-4 says she never
touches in when she joins the tube at Finsbury Park and has never been
charged extra or incurred an unresolved journey.

I can see the time in future though when they might sting travelcard users
too, or they could suspect you of evasion if your journeys come out of Z1 in
the morning and in at Z1 in the evening, never joining or leaving the system
at the other end. No doubt they want the data from people making journeys to
see where pinch points, busy routes, and where they can maximise future
price increases as necessary. It's all valuable data and if half of it's
missing it's not as useful to 'em

Cheers,
Dave