I don't see any issue with it at all. Some people do it due to medical reasons, others becuase its easier than switching from sunglasses to regular ones, and others, well, they just do.
Doens't bother me in the least. And half the time, I probably don't even notice. I wore them insidie once after going to the eye dr. as my eyes were QUITE sensitive to light, and even with them on, I still had some issue. Had I not had something that needed to be done that afternoon, I would have simply gone straight home. as it was, I did my task, then left.