You are assuming that somebody buying a new D800 will get a bad camera. You did, I didn't. Based on that small sample there is a 50-50 chance. The odds are probably better, as I do not think the majority of D800's have the focus issue.
Perhaps more sensible advice is that if you buy the camera, check it out promptly and return it for an refund or exchange if you are not satisfied.
The D800 is a wonderful tool, but a defective one is not acceptable. However, don't forgo buying one just because there is a chance that you'll get a bad copy. As they say: "Trust, but verify".