Bump what Don said. There are practically no cases to cite precisely because there are practically no machineguns to use for such cases - and the few that exist legally are rarely brought out for any reason, much less serious defensive use.
A good counter-argument would be asking why a rapidly growing number of police departements are outfitting cops with full-auto M16 variants. Cops can get machineguns and use them without wierd legal results, so why are they choosing to have the full-auto option available?
On a historical note, the 2nd Amendment - backed up by the Militia Act Of 1792 - was written specifically written with the intent that common citizens would own the standard military weapon of the day. Back then, it was a musket. Today, that's an M4 or AK-47. You ARE a member of the USA militia, you ARE enrolled and notified (via Selective Service), and it IS your duty to be suitably and reasonably prepared & trained; do you have your bugout bag and M4 ready to go at a moment's notice?
FWIW: When I got my Class III AOW, the sheriff signing the paperwork specifically said "this is for collecting purposes, right? I don't want to ever hear about it being used improperly." Same goes, at least implicitly, for all other C3 possession: the reason you don't hear much about machineguns being used for self-defense is because they're all locked away for collecting, not defense.
And yes, there is the case of the H&K sales rep defending himself and wife from a murderous gang via a Ruger machinegun.
