Honestly, I think it's a bit of a "look how cool I am" syndrome. Few will admit this, I think. (or get overly offended at my opinion, I hope. I like and use them too, so feel I can say something without total predjudice)
I love my gold mirrored shield because it's like a little happy window to view the world through. It seems to filter sunlight down to eye soothing levels just fine, but it's not dark. It seems to both block the sun, yet make everything pleasantly bright coloured-hard to describe, but maybe I have permanent Seasonal Affective Disorder...
I have had a silver mirrored shield on a previous helmet years ago, but didn't like the reflections from the inside. I found it distracting. The gold one on my Icon doesn't seem to do the same thing, or if it does, I don't find it noticeable.
I have a blue iridium one on my Shoei that is quite dark, but not as dark as a true smoke shield.
No tinted shield is smart to use at night. (though having said that, I rode home at night in a blizzard of snow a few months back and forgot my clear shield. The gold mirrored one was passable for me to see through, rather than ice and snow blasting me in the eyes and face at highway speed, I discovered.)