The value is in changing social perception of the issue.
It's a lot easier to pass legislation to make tracking illegal for users who have explicitly opted out, for example, than it is to pass legislation making tracking illegal period.
That is, DNT is a first step with more steps planned, not a complete solution for the tracking problem.
But if Microsoft derails the first step, then the followup steps can't happen, which is somewhat unfortunate.
It's a lot easier to pass legislation to make tracking illegal for users who have explicitly opted out, for example, than it is to pass legislation making tracking illegal period.
That is, DNT is a first step with more steps planned, not a complete solution for the tracking problem.
But if Microsoft derails the first step, then the followup steps can't happen, which is somewhat unfortunate.