I looked on blizz's site and didn't see anything, and I was curious who exactlly uses the glasses icon?

Spectators (or observers). They can read chat, but can't speak.

No longer used, AFAIK.  But can still be found in icons.bni

Just out of curiosity, when was this flag ever used...or is it just there in case Battle.net needs it?

I think it was suppose to be used for mute keys but they never used them

I don't think it was. I suspect the idea was to use it for moderating special chat sessions. TestBNCS doesn't permit observers to speak, but gives them moderator privileges in all channels.