I think there may be some misunderstanding about the basic science of the 400h.
All the energy used in driving a hybrid is ultimately derived from running the petrol engine; that applies even when running on batteries only. The advantage of the hybrid set-up is that energy that in a conventional car would be lost (e.g. from over-running, coasting or regenerative braking) is "saved" for re-use; thereby making for a more efficient system. The energy management control in the 400h appears to be set up to store and reuse as much of this energy as possible, so that energy is being added to and taken out of the batteries for optimal efficiency. That's why under normal driving at say 40-50 on an A road, the energy monitor screen seems to show energy going in and out of the batteries in sometimes very short bursts. It also explains why you will very rarely see all bars on the battery capacity meter lit up, since that would imply that there is no more capacity for energy storage. Instead the energy management control constantly monitors the batteries and releases stored energy (by running the electric motor(s) simultaneously with the petrol motor if necessary) thereby to keep spare capacity for storage available at nearly all times. The drive train of the 400h is set up to utilise energy from the petrol and electric motors simultaneously when required, so that the work done by the petrol motor is augmented by the elecric motor in many situations.
From this it follows that there is no particular advantage (apart from the Tesco car park fun) in trying to keep your 400h running on electric motor only. The energy used at that stage will have to be paid for at some later stage by running the petrol engine to replenish the batteries. (Don't forget that recharging the batteries by coasting down a long hill is paid for in getting the car to the top of the hill in the first place.)
The name of the game in hybrid technology is energy efficiency, and I think the 400h makes a pretty good stab at it. In relative terms, these are still early days and no-doubt there will be significant improvements in the future.
(I don't want to get involved in the discussion about road holding or drive comfort etc. My comments are solely concerned with the efficiency improvements of the 400h over its coventional equivalents.)