Two main reasons for tint:
1) Heat/glare reduction
2) Look/privacy
The first one is definitely predicated on where you live. Tesla being, at least initially, much more prevalent in CA, you have a lot more member of this site in a hot, sunny location. Now, it's a global community and there are many locations represented.
I currently don't have my MS tinted, but I bought it that way last year with the intent to tint. I've recently noticed a few small rock chips in my windshield along with numerous micro chips (seen like a starry night when the sun is on front of me). So I plan to replace the windshield and then tint all. But I too am not a fan of dark tints, so I plan to use 35% on the sides and back, and 70% on the windshield.
The second is truly a personal preference. Some folks think a dark tint looks good on their black car. The car's interior seat color is also a factor, as the light/white seats really brighten the inside so some go darker on their tint to compensate.
So it's really a combination of environmental needs and style preferences.
The newer tint technology, ceramic, does a great job of lessening heat at any LTV. So if you want a more comfortable ride on sunny days, but don't want issues driving at night, there are 70% films that are nearly clear to the naked eye, but still block a decent amount of heat.
Whether you get hot or not, UV damage is a real thing. Most factory windows have a little protection, but not much. Tesla glass roofs do have more UV and IR protection built in, so tinting that seems to be optional, more about look than heat.
Also, a lesser talked about benefit to tint is it's a slight sound deadener. Very small, but real drop in road noise into the cabin.
Tint quality matters. Thicker tint will do more than cheap. I don't recommend cutting costs with tint. You could end up with haze, discoloration, or bubbles around the rear defrost lines. And definitely do your homework on top shops in your area. Need to go where they work on Teslas.