The worst of both worlds:
(1) If you rarely use the gas engine, you are wasting electrical power hauling around a gas engine and gas tank
(2) If you often use the gas engine, you are wasting gas hauling around extra-large batteries.
The basic theory is that an ICE that only need to cover part of the driving range can be more efficient than one that needs to scale from pulling out of the drive to speeding down a freeway overtaking a lorry at short notice. Bus manufacturers were experimenting with flywheels and all sorts of crazy tech to take strain of the main engine because this hit them hardest for various reasons.
As battery and electric car tech improves and lowers in price, purr electric cars will encroach further onto hybrids, but the same forces will let hybrids encroach further onto ICE territory at the same time.
For some of us, a plug-in hybrid is a great choice, at least until the range improves on pure electric cars.
Specifically, in the linked article they are trumpeting increased electric range in the Volt. It makes more sense to get a Leaf and rent a Prius for long range driving.
The Volt weighs much less than the Tesla. The reality is that battery tech is expensive, heavy, and delicate so hybrids still make sense. A near empty gas tank weighs nothing and a near empty battery weighs the same as a full one. This stuff gets a bit more complex as you look into how it really works. Not to mention the Volt comes in at pricing that's half or even 1/3rd the price Tesla, depending on packages/trim.
The Volt is a great concept and the next logical step in hybrid technology. It leans more on the battery powered aspect, than say, the Prius, which is still primarily powered by gas. We're just not at the point where a full electric is in the cards for most people.
- Batteries are expensive and buying a vehicle with batteries that are large enough to cover most uses will often be a more expensive approach than hybrid.
- There's a lot to be said for a vehicle that isn't range-limited even if it's normally used as a second vehicle to drive around town.
Edit: That also depends on the weight of each component. If the gas engine is not expected to run all the time, it can be smaller/lighter than an engine in a comparable car. If it is just a generator, which it seems to be, then a whole lot of complexity is saved, beginning with the transmission. Electric engines are very small for their power, which also helps.
... but batteries can still provide lot of gas-free kilometers before becoming a dead weight, making big contribution to your mpg.