Dttomcat--first let me say thanks for the edit-I needed that.
Secondly, the way it was presented to me ( which I guess I should have explained) is that shelter wasn't for warmth, it was for protection. The ideal of shelter in the rule of threes is to keep the sun from dehydrating you, the rain from soaking you ( very important in some countries where acid rain exists to the point of being harmful to humans) and snow from piling up on you or your gear. You'd be surprised how fast you can lose your whole pack because you put it down to gather firewood on the other side of that crevasse.
Also, it was presented as you need to be thinking of how you'll shelter yourself within 3 hours ( in most of the classes). As far as the widespreadedness ( is that word?) of this rule, I'll add that only two of my survival classes were taught by the U.S. the rest were taught by other nations' military instructors.
David