Peanut butter has some awesome qualities--the biggest (aside from yummyness) is its nutritional density. Peanut butter packs an awful lot into of nutrition per ounce--it's nutritionally dense, doesn't require preparation or refrigeration. As an example, a 1.5 ounce packet of MRE peanut butter contains 250 calories, 21 grams of fat, 10 grams of carbohydrate and 10 grams of protein. For just over a pound of weight, you can carry 11 packets of MRE PB--giving you 2750 calories, 231g of fat, 110g carbohydrate and 110g of protein!
In comparison, a typical Mountain House meal-in-a-bag weights around 8 ounces and contains approximately 600 calories, 16g fat, 80g carbohydrate, and 30g of protein. Two Mountain House meals (about a pound--not including water needed to cook and rehydrate them!) would have only about 1200 calories, 32g of fat, 160g carbs and 60g of protein! Peanut butter wins on all fronts except carbs.
Aside from dense nutrients, peanut butter requires NO cooking, heating, re-hydration or refrigeration. You can eat it on the move, spread it on crackers or bread and enjoy. Since it requires no cooking, there's none of the associated cooking gear, mess, fire or smell. That's ideal for a bugout, where you will probably NOT have time or a secure location to stop, cook and eat. It won't melt in hot weather. It works as an excellent bait for traps. Finally, its not some weird exotic survival food, but something that most of us have eaten since childhood. That means that you'll eat it in a non-emergency situation--thus making sure that it stays rotated and current--and it also means that your kids will eat it without out a fight. My little son--currently a very picky eater--even loves the stuff--here he is swiping a packet out of my Camelbak.
NOTE: Sopakco recalled their civilian MRE-style Peanut Butter packets during the salmonella scare last year. You can read more about this here: http://www.mreinfo.com/civilian/mre/sopakco-sure-pak-12-mre-recall.html It doesn't look like the military packaged Peanut Butter packs (like those in the above pictures) were affected.
It looks like Emergency Essentials has 'em in stock for .50 a piece (a pretty good deal); found here.