I agree, but do know one trult devious and workable exception.....

One of my old friends and his son have the same name, first, middle and last, and have shared the same FF# on a US domestic "legacy" (un-nmamed to protect the malefactors from possible discovery) upon which they both fly with some frequency (simply never simultateously). I'm not sure how they've managed to continue the flim-flam over a couple of decades, but both seem to be able to exercise the rewards. I suspect that living in the same toiwn helps, but obviously they can use only a single address and e-mailbox.

I can't remember just how definitive and personally descriptive FF information held by the airline's really is....