Play around with NUKEMAP:

http://nuclearsecrecy.com/nukemap/

If you are hit directly, you're fucked no matter what. But the fireball has relatively small radius compared to the other zones.

Ducking underneath something solid is to protect you if the building collapses, which is likely to occur in the large air-blast radius. This is much like ducking under a desk in an earthquake.

Cover will save you if you're within the larger thermal radiation radius. Even clothing can be enough to protect you from burns, so any cover you can get is good.

Cover is extremely important if you're outside the immediate fireball zone (where you're just going to be vaporized). You shouldn't infer from the fact that bunker busting bombs successfully bust bunkers that attempting to survive a nuclear explosion at some distance from you is pointless.

Have you ever looked at pictures of the aftermath of Hiroshima? People who weren't behind cover survived the immediate blast but got horrifying burns.

The mocking is utterly unjustifiable. Duck and cover would have saved millions of lives if MAD had come to its conclusion. The inability of a building to resist bunker busting bombs is completely unrelated.

It could. If you’re in the inferno, no. If you’re outside the thermal flux, protected from ionising radiation by simple structure (i.e. away from windows) and within the pressure flux, duck and cover works. If you’re in cases one and two, you die, quickly or painfully. Since you don’t know ex ante where the bomb will burst, duck and cover makes sense.

It's going to make a huge difference is the blast is far enough away for you to not be in the "obliterate everything" zone, but still close enough to be dangerous.