It's an absolute waste of time for the majority.
IF something happens to cut you off for three days - no power, can't leave home etc - well we'll have three crappy days. We'll dig out the bag of Xmas tealights, have early nights, wrap in blankets, eat tinned stuff from the cupboards, drink the water we always have in anyway. Then go back to normal.
Anything more drastic than three days, well, following the guidance is only going to delay the inevitable PITA anyway.
Anything seriously drastic - not quite nucleur but long term life or death type scenario, due to society ending and everyone having to fend for themselves for weeks or months or forever. Well. Unless you're a serious prepper that is remote, hidden and can remain self sufficient forever, you'll either be quietly dying if unfortunately you're incapable of fending for yourself - or you'll be out smashing supermarket windows and burgling homes in well to do areas to feed yourselves, with the rest of us. Being able to stay exempt for the first three days of chaos is unlikely to have much impact on your long term survival.
Anyway...whichever way you look at it, prepping for three days will only be worth the effort in a scenario where the alternative is minor inconvenience. I'd rather save myself the effort and deal with the inconvenience if it arises.