Actually, this is a common misconception.
If I travel to London for instance, a journey I take several times a year and approximately 240 miles (I do it in one non-stop trip of course). I can increase my fuel efficiency by many mpg if I fully charge my PHEV first and - this is the important bit - use the Sat Nav. This enables the car to meter out the charge to maximise its efficiency. Cruising on a motorway is a weak point for EVs, whereas in heavy traffic, it plays to their strengths. My PHEV can keep some charge available for when I get to London, for instance. It works well and the car usually depletes its charge with a mile or two of my destination. Without using the Sat Nav the car tends to use the charge at the beginning and also on the motorway, which means you drag around all that ruddy extra battery weight for no benefit. Once in London I'm generally screwed, as they say, as charging becomes a nightmare and is generally not feasible, so the return trip is done in pure petrol mode. I've explained the scenario before, so won't bore with my life story on this matter, again.
PHEVS are the most complicated of all cars and you have to give them as much information as possible, if you want to maximise their efficiency. I would never buy a secondhand one, they are far too complex. For instance, my car can run with ..
Front EV motor
Rear EV Motor
Front and rear EV Motors combined
Petrol engine
Petrol engine and front EV motor
Petrol engine and rear EV motor
Petrol engine and both EV Motors
it can recharge from...
Type 2/mains supply
Front EV motor
Rear EV motor
Brake regen using a combination of the above
or the petrol engine can recharge as you drive (you can set for 3 different levels of total charge).
It can run in
'Pure electric',
Pure petrol
'Hybrid' combination
'Sport' where ever thing can run simultaneously
'4x4' where the electric motors run almost all the time and the petrol occasionally helps out.
Nuts! Dread to think the problems some poor sod will have with this car in 10 years time.