If we could be absolutely certain the named earliest ancestor of every test subject was correctly identified, the need to know the entire lineage would not be so important, but consider this scenario…

Two testees claim the same earliest ancestor, but their test results don't match.  [We have several such cases in my projects right now, each of which will require testing of more lines to resolve.]  What do these non-matches mean?  Obviously, in each case one (or both) of the testees has made a bad connection or has an adoption or hidden paternity, literally "somewhere along the line."  So…  They can compare notes, right?  And find the mistake, right?  Well, maybe.  But in the meantime, the rest of us are left in the dark and, most importantly, what if one of the testees passes away?  [There are already several cases in my projects where the test subject has passed away, though in each case, thankfully, the contact person was a younger family member and is still living.]  What if we can't find a living family member to give us the testee's lineage?  (Keep in mind that the testee's identity cannot be revealed, so the project administrator is severely hamstrung in trying to find a living family member.)  Without a dependable lineage, test results can no longer be meaningfully interpreted, and the person ends up having been tested essentially for nothing.

Please keep in mind that the testing you do, today, will be useful to genealogists decades, even centuries, from now.  Not knowing the complete lineage of the test subject may someday render the data ambiguous, if not useless.

Probably a majority of us having the time to pursue genealogy are retired, if not elderly.  Anyone who has been working on their genealogy for years has lost friends and fellow researchers — and seen their work lost, too.  If we lose a member and don't have his lineage, we might as well toss the data for all the use they'll be.

Why do you want the lineage all the way down to the testee?  About 10% of people tested turn out not to be who they think they are, that is, there is an unknown adoption or hidden paternity (illicit relationship) in their lineage, that is, an NPE.  It's important for the project administrator to know, or at least be able to deduce, where the "break" in the line may have occurred.

Another consideration is that, as project administrator, I do my best to authenticate (document) the lineage of every member, so that you can be confident in viewing the lineages that they are accurate and won't mislead you.  If I don't have the lineage, I can hardly authenticate it. 

Lastly, there is one consideration that probably wouldn't be obvious to anyone without a web site.  The size and popularity of my genealogy web site (over 15,000 pages receiving several thousand page hits per day) generally places it near the top of a hit list at Google when someone searches there on a name represented here.  We need the family group sheets as "fodder" for the Googlebot to index to insure that a search there on a name here finds the DNA project.  When arriving here, finding a wealth of useful information — and a spirit of cooperation and generosity — will encourage participation and further sharing, which is crucial because the success of this project and the probability of members finding a match depend in large measure on being able to attract more participants, ones who also supply their lineage.

Genealogy is like a jigsaw puzzle, with no picture on the box to guide you how to put it together.  Everyone has just a few pieces, which alone don't reveal much.  If everyone shares their pieces, we can put together enough of them to reveal the image they contain.  The more pieces people contribute and the more people contributing them, the clearer the image gets.  If everyone hides their piece of the puzzle, none of us will get anywhere.