My own experience is that one can sort of get addicted to wearing them. When I was younger, I would wear them all the time and I am sure my eyes were far more light sensitive. The last ten years of so, I have simply not bothered (mostly due to losing them one too many times) and find I now rarely need to squint due to glare. I am guessing that letting sunlight into the eyes (not, of course by staring straight at the sun) triggers various desirable hormonal/chemical reactions while always having shades on might interfere with these natural processes.
Is sunglass wearing just another way we create an artificial lighting environment that might be messing with us in an undesirable way?