Once you go back 6 or 7 generations, then yes, the DNA is not really telling you anything. If that person was indeed a common ancestor, you only have a teeny fraction of the parts of the DNA that ancestry is looking at, and the same applies to your cousin. The chance that you and your cousin have that same teeny fraction is pretty slim. Here are some numbers from the ISOGG wiki:http://www.isogg.org/wiki/Autosomal_DNA_statistics
The numbers really drop once you get past 5th cousins. For example, on average you share only 0.00305% (that's 3 thousandths of a percent) with a seventh cousin. I would also bet that the information that leads you back to Halehumdra came from the same source that your distant cousin used.
The worth of the DNA project has to do with your goals. If you intended to use it to help with ancestors from the year 1100, then yes, you've wasted your money. Even though you can take a particular line back to that Prince or the 23g grandparent, I'm sure you have gaps or dead ends 4 or 5 generations back on some of your other lines, and DNA can indeed help with that. These cousin matches you are seeing 10 generations back could very well be from those much closer gaps. The point of moving the slider back is those matches could be closer than ancestry predicts.