Whilst the single for almost all journeys is £2 and multi operator daily and weekly tickets are excluded from tap and cap i can believe there isn't really much interest in using it!
Where I am there is a £14 cap for weekly travel within the city zone so, given I could make 14 single journeys a week on routes which go out of the city (I usually end up using the bus at the weekends), I definitely have interest in tapping off even with the £2 max single fare!