said by sivran:...It'll be another datapoint in the tracking profile they build. 
Then they'll use it to try to sell you privacy snakeoil! 
It's a higher-value datapoint as well. I suspect most folks don't take all that many privacy precautions, and DNT "says something" out of the norm about folks that do. I'll leave it to the professional data-miners as to what it might directly imply - but it seems to me it definitely would at least be a significant data-bit in the overall "snapshot" a tracker can assemble from a user's browser agent and JavaScript responses. The more unique that "signature", the better it will act like a universal tracking cookie.
I browse with JavaScript shut off, which is its own rather out-of-the-norm signature data-bit in a universe of heavy JS users - but that data-bit is far more than balanced by all the other bits that shutting off JS denies a tracker.
