If you burn gas or diesel, it's very clear how much CO2 gets emitted, so you can measure fuel efficiency in g CO2 / km.
It's impossible to do this for EVs because the number depends very much on how the electricity is generated: whether from your rooftop solar or from the coal plant down the road. Similarly, electricity rates vary.
For doing a comparison, the MPG-e is not an unreasonable measure. (Just don't mess with the number!).