This is one of the the design/engineering trade-offs/wins for a current (gas) hybrid depending on your driving regime.
If it is lots of urban stop-start driving, then storing energy from regenerative braking as you stop, killing the engine so it doesn't idle, and starting on stored energy before restarting the engine makes sense, and is a win vs just having a (gas) engine.
If your driving is mostly cruising on the highway then adding all the extra mass, complexity and cost vs a simple diesel could be considered a fail
One of the biggest wins is providing feedback to the driver as to their instantaneous/cumulative fuel consumption (e.g MPG in big digits, right in their face). Curiously some people then modify their driving style to minimize consumption/maximize m.p.g when presented with such information - e.g. the hyper-milers. Maybe presenting this as cost per mile, or cost consumed would provide an even bigger incentive.
Or there is the "Dodge" way - why invest in economy oriented R+D when you can buy customer loyalty with a "$2.99 Gas Guarantee". Just don't try thinking about the depreciation on residual value when it comes to an end. Ouch.