If I spent a lot of money on solar cells, I would want them working whenever the sun shines and I would want them to have the optimum orientation. That means they would need to be stationary, not attached to the car. I also want to keep my car in the garage.
It could be useful for charging during the day at work and also work as a sunshade to keep the car cooler Not sure how many kWh it could put in the in a workday though.
I park my Leaf under my Solar Grid...and the outlet on one of the legs is where I plug my car into. The grid has been up since 2002 & is still going strong.