In message , Tim Roll-Pickering
writes

Yes but it's the "everyone must always touch in and out" problem. It could
be particularly messy on routes with semi-fast services and stations with
multiple routes to London where you don't know for sure which service you'll
be getting until it actually gets onto the platform.

I still don't see the problem. You touch in at the start of your
journey, touch out at the end, and Oyster charges you the PAYG fare from
your starting station to the boundary of the outermost zone covered by
your travelcard. Exactly which route you take shouldn't need to come
into it.
--
Paul Terry