It does seem, though, that for the vast majority of commuter journeys in the South East where the traveller isn't entitled to a Railcard that can be used at any time, single+contactless+contactless+Railcard single is going to give a significant saving.
From Ely to Kings Cross, to travel at peak hours in the morning, an anytime day return is £55.30. An anytime day single (£34.60) and a Network card day single (£22.80) is £57.40. There is no off peak single.
The anytime travelcard is £66.70, so most of the time will be more expensive than an anytime day return and a few contactless TfL journeys.
You can save a bit if you split tickets at Letchworth (anytime day returns for Ely-Letchworth are £18 and Letchworth-Kings Cross are £30.70), for a total of £48.70. That isn't too bad on a normal day as most of the trains I wanted to use stopped at Letchworth. Its a pain when there is disruption, and an Ely-London ticket without a split at Letchworth would have let me use the Greater Anglia trains to Liverpool Street.
You can also save a little with splits at Cambridge, and then using only Greater Anglia trains between Ely and Cambridge. Greater Anglia-only tickets into Liverpool Street are quite a bit cheaper, but slower.
Off peak, and with a Network card, the Travelcards are better value.