There probably aren't any weight savings in the hybrid configuration.
But you would eliminate gearboxes and all the headaches that come with that. You would eliminate the whole anti-torque shaft, head and actuation. You could do startups in no time etc. And it would be much more reliable as electric motors pretty much never wear unless the bearing goes.
The fossil fuel debate is interesting. However, even though your electricity might be fossil fuel based (not always, but likely) it makes much more sense to ship all the fuel to a power station, than to truck it out to every nook and cranny of the world to fill it up into aircrafts. The infrastructure of electric delivery is already there, it's paid for and available pretty much everywhere. You can refuel anywhere. Obviously as electric cars are already becoming quite the reality, I'm sure there will be some kind of "refueling" standard in place for them. Perhaps aviation could use the same connectors and infrastructure?