You want variety for a number of reasons, but those with substantially more than a week or so of food might want to keep track of how many 2,000 calorie days of food they have, since that's about what an average person needs. A pound of spaghetti (1600 calories) plus one 15 ounce can of sauce is about 2,000 calories. Similarly, a pound of rice and something such as canned soup and tuna to stir into it adds up to 2,000 calories. I have lots of canned meats and fish for the complete protein (and it's the stuff we eat a couple of times a week, so we're used to it and we like it), but a little over half of our emergency food calories comes from grain. The 20 pound sack of rice I just got for $6 will, with 20 additions of 400 calories each (averaging $1 per can), provide food for one person for 20 days or for a family of four for five days. If nothing goes wrong, we'll eat that rice late next year or early the following year, and replace it with another sack of rice, but in an emergency, my family will not go hungry.
There are a lot of people (including myself, unfortunately) who would be able to get along well on much less than 2,000 calories per day for an extended period of time, if need be.