I would actually like a solar roof for just this reason: when I have no choice but to park in the sun, it would be convenient if it could produce enough energy to maintain a reasonable temperature without impacting the remaining range.
The best way to spend money improving an EV is almost always going to be on batteries or weight reduction. Self charging is neat, but unless you get much more efficient solar arrays, it’s always going to be limited to niche applications. Five miles more range is always going to be useful, solar arrays which add a mile or two of range under specific conditions are far less interesting.
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.
I probably come across as being more anti-solar roof than I am. Mostly just feel like this isn’t a feature I’d make a purchase decision on and wouldn’t likely pay too much for.
I was reading on the Tesla forums about just bringing a portable solar array for camping and it’s weirdly a lot more complicated than you’d think. Perhaps having it integrated would make it simpler, but it’s a lot trickier than just topping off a 12v battery with a solar array.
And if your only goal is to use it as an excuse to talk to your friends about having solar panels in your car then a small panel makes even more sense -- you can talk almost as much about a small panel as a large one for a fraction of the cost.