Not many now, but if websites start feeling the squeeze from ad revenue, it will become more common (see Hulu making commercials 2-3x as long if Adblock is enabled).
It's also an unfair comparison because Adblock is opt-in -- you have to download the add-on to get it to work, which not many people are willing to do (what percentage of Firefox users use AdBlock -- maybe 5%?). DNT, on the other hand, is something all Firefox users will have enabled by default -- Imagine 20% of the worldwide browsing market suddenly blocking all tracking cookies.
It's also an unfair comparison because Adblock is opt-in -- you have to download the add-on to get it to work, which not many people are willing to do (what percentage of Firefox users use AdBlock -- maybe 5%?). DNT, on the other hand, is something all Firefox users will have enabled by default -- Imagine 20% of the worldwide browsing market suddenly blocking all tracking cookies.