It's more of a regional thing. East coast weddings tend to be more formal. Here in the West attire is almost never stated on the wedding invite, but when it is, I view it as a suggestion, not an order. After all, not everyone owns a tux or evening gown. Sometimes the wedding invite will give hints about the wedding theme like "outdoorsy" or "western" or just "casual". This doesn't mean you have to wear flannel, checks or bluejeans, but it would be nice if you did. However, if you are in the bridal party you will be told what to wear and you better do it or risk bridzilla's wrath!