Fading is all about photons and the photosensitivity of various organic pigments. For me, keeping a guitar cased is more about maintaining playability, pots and switches, as I’ll discuss below.
Indirect light can still fade a finish; it just takes longer. Remember, if you can see the wall, it’s getting plenty of photons, or you wouldn’t be able to see it. Even light bulbs can eventually affect a finish.
I have a Northern Lights PS. It stays in its case until I’m going to play it. There is no fading whatsoever; it’s as vibrant as the day it arrived nearly 4 years ago, but I keep my guitars cased for other reasons: the case buffers temperature and humidity changes that occur at different times of day, on different days, etc., so my guitars don’t need seasonal setup adjustments. So it just makes life easier for me to case them.
If you’re curious about what happens in a typical room re: humidity and temp changes in a single day or week, get a cheap digital hygrometer and take note of the readings for a while. You might be surprised.
Also, in a home, things like evaporated cooking oils and other airborne gunk are spread around as the air circulates. This is why pots eventually get scratchy, why dust that lands on the instruments turns to gunk and goo, etc.
So they just stay nicer to play if they’re kept in cases. If you prioritize having a great feeling instrument to play, it’s gonna “keep” better in a case. I also toss one of those D’Addario humidipaks into every case, to keep the inside of the case at 45% relative humidity in all seasons (it both absorbs and releases moisture depending on RH); the PRS factory’s HVAC system is programmed for the same RH according to one of the PRS folks I talked with (I’d guess that’s except for the wood drying room?).
I’m speaking only for myself here; if I’m spending PS money for something to hang on a wall, I buy a beautiful work of art meant for a wall. If I’m buying a musical instrument, I want it to be in the best possible shape to serve as an instrument. YMMV, and I’d never tell folks they need to share my value system!