A dead battery is just dead weight, imo.
Nah - it's an energy accumulator. It allows you to carry a certain amount of energy from an alternate (and cheaper, cleaner) source than the fossil fuel in your tank, and mete it out to the road via a type of motor that's both far more efficient in use than a reciprocating piston device, and doesn't use any energy at all when "idling". Typical internal combustion engines lack such a thing other than the relatively tiny flywheel on the crankshaft (the fuel tank doesn't count, as you never refill that whilst in motion...), so they both have to be large enough to cover occasional, short, peak transient demands even though that tends to make them overpowered and inefficient for general cruising (if they're not, then they feel "underpowered"), and keep running continually at a certain minimum level of output (enough for a good 20-30mph) and fuel use when only a tiny amount of power is needed in short bursts (slow moving traffic queues). Hybrid systems cover for this, by adding said accumulator. The engine can be smaller, running more in it's most efficient mode, with the excess power at that point being syphoned off to use either to boost the peak output, and/or to be trickled out with the generator turned off when it's minimum output would still be quite excessive.
Said energy coming either from the electricity grid, your own renewable generator(s), the onboard engine (taking additional energy from it when it's operating in its most efficient modes in order to then spend it via the electric motor with the ICE turned off, when it would otherwise have to run very inefficiently), and from using the motor as a braking generator instead of spunking all your accumulated kinetic energy away as heat.
The problem we have is that current battery tech, whilst very much improved over old Lead-Acid types, is still very bulky, very heavy, slow to charge, difficult to thermally manage and of a limited lifespan vs a traditional galvanised steel tank full of liquid hydrocarbon. And although there's still work being done to improve it and the other alternatives (flywheels, supercapacitors, etc), no-one looks like they're on the verge of a revolutionary breakthrough anytime soon. Carbon-hydrogen bonds just seal up so much more energy, and release so much of it so quickly when burnt, that all the alternative methods have a long way to go to come close to matching it.
And of course having two storage and propulsion systems on-board is always going to compromise the interior room and handling of a vehicle - unless you can package it very smartly. Which Vauxhall/GM came pretty close to doing, but they still ended up having to shove an amount of the batteries in a central propshaft-like tunnel that means only four passengers will fit into a 5-seat-sized car. Now, maybe if they were to trim that 1.4L engine down a bit ... say to 700cc ... allow it to run a little harder when the batteries were depleted ... and shove the centre-tunnel cells in place of the deleted cylinders ... it might then seem like more of a normal car.
Or, bloody hell, just make it run on H2 already and let's forget the whole silly business.