You think ad networks will be the one who honor DNT? The very same people who profit by tracking?
If few enough use it, I think the "serious" ad networks will honor it. Google might, for example.
Frankly I think the whole thing would be better if adblock was just installed by default in every browser.
So do I. That's not really what the discussion is about, though. DNT is a compromise between users and advertisers that may work for both. This is the industry's attempt at self regulation, before they get laws shoved in their face.
Ads are nothing less than visual pollution. Tracking is also one of the reasons that we have cookies and all the other security problems with the web. HTTP was meant to be a stateless protocol and should remain so.
Ironically, you are logged in - using a cookie - while posting that ;-) Truth to tell, I agree with you, but that train is long gone. HTML/HTTP is a weak basis for application building (this I say as a programmer who has written both desktop- and web applications), but there you go - every application that's written these days is written for the browser. Would I have preferred it differently? Yes. Is it a reality we have to live with? Yes.