What is harmful is being indoors all the time, and getting out so rarely that you get sunburned in short amount of time. Far more damage is done by being indoors too much than being outdoors too much.
The white skin has evolved because the body needs sun and UV light to function, one would die otherwise.
This is an extremely popular simplification. The scientific jury is still out on this one, as it seems that paradoxically people with constant exposure to sunlight are less likely to get skin cancer as compared to those that get sporadic exposure.
Also MS is extremely prevalent at high latitudes compared to the tropics. Why? We know vitamin D has a role in protecting the insulation of the nerves, so maybe that's the mechanism. Anecdotal, but the one person I know with MS is white as the snow and avoids the sun like it's the devil.
It's unfortunate that exposure to UV is so harmful, I'm left wondering if trials using UV lightboxes to stimulate vitamin D production would change the kinds of outcomes (cancer, heart disease, depression...) we're expecting it to .