DNA Circles also include strong matches to your matches that may not match you.
In other words, you match 6 people who all, like you, have John Smith, b. 1750, identified as an ancestor. However, 6 other folks identify John Smith as an ancestor as well, but they simply didn't receive the same DNA you and your matches did - or they are sharing some DNA with your matches, but not with you. While you are genealogical cousins (meaning you have paperwork that verifies your relationship), you are not genetic cousins (you do not share DNA). My mom, my brother and I have all been tested. There are hundreds of folks that match my brother and my mom, but not me and this only because that particular DNA was passed to my brother, but not me. They are still my cousins, but genetically, we have nothing in common.
The only way to be certain that you are sharing DNA with descendant of John Smith, is to ask your matches to participate in GedMatch, analyze the chromosome browser and identify a common chromosome and segment that you share. The folks you match can and will likely have overlapping shared DNA segments with other descendants of John Smith that was not passed along to you.
The DNA Circles, in my opinion, are valuable. Even if they are a lite and generic attempt at accomplishing what a chromosome browser does, I find it very cool to see 18 folks sharing DNA all pointing to the same ancestor. DNA Circles have verified my descendancy from my great grandmother's line. Her mother was married three times, and my great grandmother never appeared on a census sharing the same name with her mother, nor did my great grandmother appear in any of the well-researched and published lines of her family.She was nearly lost to history until I was able to find mention of her and her mother in a county history and I was able to connect her to the family I suspected she belonged to. Subsequently, of the 12 DNA Circles that appear in my results, her line is amongst my strongest.
As more people get tested, your DNA Circles will grow with more genetic matches, and their genetic matches as well.