Not sure, but I think a German court(?) recently ruled it's a legitimate way for a user to express intent that should be honoured by website.

Technically there isn't a reason to look for other means, the reason is mostly that advertisers would rather not have visitors choose this option. The reason DNT was almost instantly rejected, for example, was that the compromise text mentioned the user enable DNT, but then some browsers enabled it by default. Advertisers: Hurray! We found a reason to ignore user preferences!

So, for all I care it might be DNT, GPC or a plug-in that auto-clicks REJECT COOKIE. It's the default I would expect. It would be great the web simply would not offer ad targeting unless I explicitly enabled some tracking beacon because I sincerely like targeted ads better than old-school billboard.