Just to tack on a point to the sensible safety issue - of course, not everyone lives somewhere very sunny. Some folk wear sunglasses indoors and at night - I don't think they pick Prada for driving or flying a helicopter.
If you're in Australia, for example, you need polarised sunglasses - especially for driving at certain times of day (a fatwa on silver cars!). You need them less so for an indoor shopping mall.
They're fashion accessories is what I'm saying - so rep sunglasses can be good from that point of view.
And, of course, if you're buying a lady glasses to get laid - damaging her eyesight may actually help you