They're touted as a partial replacement for the weekend travelcard (which gave a 25% discount over the cost of two one-day cards) but...
- There are only two kinds: 1-2 and 1-6
- The 1-6 provides zero saving (£18 vs £6x3 off-peak, or £36 vs £12x3 peak - weekdays before 9:30)
- The 1-2 only has a peak version (but does save you money: £15 vs £6x3)
- The leaflet makes the point that because Sat & Sun are off-peak days, you'd be stupid to buy two peak cards in a week: it'd be cheaper to get single day cards instead for one of them
To add insult to injury, the LT cards (allow peak-time travel on tube, bus, DLR etc but not trains) are going: everyone who can get to work without using the train is going to be charged more anyway.
I presume they've still not implemented the promised feature of Oyster cards whereby if you buy enough single tickets, it will start behaving as a travelcard thus saving you money...