They're not—as long as the PV cells are a supplementary charging solution, in addition to wall charging, to the batteries. You'll get a bit more range out while driving, especially when the car is a lightweight low drag design and PV cells may be the only thing needed to keep the constant 90 km/h speed in a sunny day. And when not driving the cells might be enough to get the 10...20 km or so commuting range back over your 8-hour workday.
But putting PV cells on a 3 ton electric SUV or pickup truck is stupid, it won't do jack all due to the inherent inefficiency of such vehicles.