> I would still argue that a null value is arbitrary and does not map to the user's wishes.

It's not arbitrary though, it clearly spells out that the browser vendor doesn't know what the users intent it.

Again, fair point - but then by definition the second clause of my statement is correct - the user's wishes are unknown, so any direction taken is then... arbitrary? :)

At any rate, you seem well-versed in this area, so let me ask you a question: what is the difference the website's behavior between a "null" and a "track me, please" value in the header?

It's not binary, it's ternary. 1 opt out, 0 opt in, null unknown choice.

In practice the difference between null and don't track me please are probably non existent. However, in the future if this were to take off it's possible that someone would come up with creative benefits/uses for tracking that provide incentive for users to be tracked.

edit: Don't think I addressed your question. The difference between null or tracking ok and don't track are likely just to be generic advertisements shown to a user instead of targeted ads, the prevention of some back end selling of user interaction data and some other things that most people have no idea is going on. If you can't track users across sites you lose some of your ability to build up profiles for them. Advertisers will argue that the ads will have significantly less value without those secondary or tertiary ways to monetize eyeballs and I suppose you could see some decline in what advertisers are willing to pay for impressions. Ultimately I've been removed from advertising for a few years so I'm not entirely sure how much it will make a difference.

This is really helpful - thanks.