They CAN be helpful to your guests. A friend who was coming to our Spanish reception was planning on wearing the dress on the left. I told her to wear it if she really wanted to, but most people will be in cocktail or evening dresses (which was definitely the case). She was annoyed at the time, but figured out something else to wear (right) and you know what? She thanked me for not letting her wear the other last night, said she would have felt underdressed and uncomfortable. Just saying, dress codes can help some people!