McDoubles were popular with skinny guys who had a hard time putting on weight. One would stuff their faces with calories and hit the weights in order to do get bigger and it would be very hard if a guy didn't get in enough calories. So that's why a calorie dense item like a McDouble would help. However, that's changed with McDonald's increasing prices such that you aren't getting protein bang for your buck. Now, as one poster had already stated, you get a better deal (monetary and nutritional) with chicken, milk, lentils, beans, whole wheat pasta and eggs.
Because that's what the word means? "Nourishing; efficient as food." Obviously what efficient means is different in different contexts. They mean not going hungry or starving to death. We no longer think of that as an issue of course, and tend to worry about other food issues, which is their entire point.
I don't think the McDouble is truly the cheapest per Calorie. Perhaps they man taste/desirability which is more subjective.