I agree that the correct form is not to put a dress code on a wedding invitation but different cultures and strata of society within cultures will interpret proper wedding dress code differently.
Here in the UK morning dress might be expected but black tie almost never worn, other cultures may have a completely different form of normal wedding wear.
Personally I would not put a dress code but tell the guests what people will be wearing, after all if you are not going to speak to your guests sometime before the wedding why are you inviting them?
That's kind of the problem in the US... there is no standard of interpretation. No one knows what to expect. Maybe it's different in the UK. The question of dress code is on every wedding web site and in every wedding planning book because it's very confusing now.
Regarding the last point, many people invite friends of parents, distant relatives, etc. There's no way to talk to everyone before the wedding unless you have a really small guest list. (Ever been to an Indian wedding? 300 guests is tiny, 600+ is common.)
I think it's worse form to assume that a guest isn't able to rise to the level of the affair and therefore avoid inviting them. Besides, as BlackTieGuide.com says, Black Tie Requested is the perfect fix to this situation. Guests who have dinner jackets know they should wear them, and guests who don't know that a normal black suit with a normal black tie will be perfectly sufficient.
I never said that. I said you should not specify a dress code you know most of your guests will have to shell out money to abide by.
Based on my fiancee's extensive experience performing at weddings, the reality of the sitatution ("state of black tie") is that if you specify "black tie optional", you* are going to get the wedding party and a handful, if any, guests in BT. Hell, she even tried to discourage me from wearing my tux to a BTO wedding because of this, but I insisted. If you're okay with that as a host, go for it. Personally, I don't like the idea, and I would rather have everyone on the same level of formality.
* You = average, middle class American, which is the perspective I'm writing from.
Even "black tie requested" isn't clear to people. Just Google that phrase and look at some of the wedding forum threads full of confusion and outrage.