I've seen far too many stores and clothing websites displaying white dresses to wear as a guest of a wedding (i.e. Nordstrom, Saks, etc). Yes, the dresses are short, but THEY ARE STILL WHITE.
I get that a guest is an adult and that you cannot tell them how to dress. And that pointing out a rude guest is not worth the fuss or trouble, and would only make the bride look bad. I also know that no one is going to confuse them for the bride. But that's not the point of the no-white rule....it's just that it's rude. White is the bride's color.
When I see guests at weddings wearing white I think: either they are uninformed or don't care, neither of which are good things. And it makes me want to go throw a huge sack over their dress.
Ok, end of rant