It protects the car, it’s a way for the greater battery drain an electric car probably has to be offset. It’s like carrying around an extra tank of fuel. It could genuinely save lives whilst electric cars develop. Soo many reasons yes vs weight gain.
Edit: even weight gain is going to be negligible if the panel is implemented like a sunroof and see through. That would be lighter than metal and only the solar charge controller is added weight. Considering there are controllers for charging anyway this maybe less than I imagine.
Light weight panels tend to produce less power. The best ones are opaque and have a thick glass layer on both sides of the circuitry - so they’re not super light. Also the glass would need to be strong enough to handle rocks/etc as well as provide structural support in a crash (even if it’s just with the initial impact and then shatters similar to a car windscreen).
They could theoretically be light however in reality heavy panels might be a better choice.
But yeah I agree you’re on the money with protecting the car. This could make a big difference to the usable life of the battery by keeping it closer to the optimum charge level especially with a normal daily suburban to city commute.