Disclaimer #1 - I have a medal (2 actually) from a race I did not finish. The Chicago 2007 Deathmarch (aka the 2007 marathon, the one that got cancelled mid-race) pretty much gave them out to everyone. I won't wear them, nor do I claim them - my kids play with them, and someday I plan on donating them or something. The shirt for the race has never been worn by me, and once I find it, it is getting donated. I won't wear it, since I did not earn it.
My concern with something like this (aside from the not-insignifiant issues of age group awards, insurance, etc) is that I could give my bib to someone younger and faster than me, and thus make it more likely that I could "earn" a slot into Boston. Likewise, if there is some octagenarian that really wants into Boston, I could run a marathon with their bib, and gain them entry.
FWIW - I think there needs to be a system to allow for an exchange of numbers, though perhaps with a hefty penalty, but anyone caught running under another number (or wearing two chips/D-tags) should get at least the Rosie Ruiz treatment.