I agree that in many cases an extra dollar of battery capacity provides more utility than an extra dollar of solar charging.
But I did have one of those niche applications a while ago: I used to travel for work and often left my car parked random airport lots for months at a time which meant I occasionally came back to melted cassette tapes and a dead battery. In that case $100 of solar panels to run an exhaust fan and/or trickle charge the battery seems like it would have provided more utility to me than $100 of additional battery capacity.
I wouldn't be willing to add thousands of dollars worth of solar panels like the Sion and even if I did I wouldn't consider it a replacement for charging via the electric grid. But if my next car comes with an optional/after-market "solar package" that can integrate solar panels into the roof that seems like it would provide some value to me, at least more than the fancy wheel or audio packages they currently offer.