Yes, that's the effect. But I'd only have to carry the generator portion when needed for a road trip. You could perhaps even rent it only when you need.

Even if the generator was in a small trailer, that would provide the range freedom without loosing the storage space (and help with venting/cooling/noise).

> Even if the generator was in a small trailer

This has been done; but the small trailer also adds significant friction and aerodynamic drag. It also adds complexity in driving and maintenance.

True, except the road trip case is also the case where you want more cargo capacity than is standard. A rental trailer for one week that lets my wife pack the kitchen sink actually makes sense.

At that point might as well just rent an gasoline engine car.

Except the engine isn't always there as taking up extra space and weight.

Weight probably isn't a problem - a Chevy Volt weighs a lot less than a Tesla S.

You don't even save much by keeping the gas tank mostly empty when you aren't expecting to need it -- the gas weighs ~6 lbs / gallon, so you might save 50 lbs by keeping the tank on 1/4 when you aren't planning to drive past the battery range.

Think about it this way: batteries are really heavy, and the Tesla has like 5x more battery capacity than a Volt. Meanwhile, if you're mostly making short trips, you're hauling around 3/4 of those heavy, heavy batteries as a contingency.