Another reason that a gasoline-electric-battery-motor-road path makes sense is that it is easy in the future to replace the gasoline engine with some other sort of power source, without having to do as much re-engineering of the electric powertrain side of things. For example, if GM wanted to package a diesel range extender for Europe, or drop the range extender completely, it would not be a radical re-engineering job. Perhaps in the future, the gasoline I4 will be replaced with a hydrogen fuel cell stack, or a flux capacitor.
Having no mechanical connection also allows for some packaging flexibility. Though GM decided to package the 1.4L I4 in the traditional underhood location, in another application it might make sense to package the motor in the rear of the vehicle--Voltec Fiero, anyone?
*Props to Sam, who is an actual engineer, not just a news writer, he tends to understand his topics.