The reason they created the list was to help their members be prepared for accidents and emergencies. Could you take more? Of course, but these are the bare "essentials."
The original list has been updated, modified and expanded by several other organizations through the years. Here is my version:
Every outdoor enthusiast has his or her own opinion about the best devices for starting fire. Because fire is so critical to to survival, I prefer redundancy. Carry at least two fire starting kits and keep them in separate places to reduce the possibility of losing both.
The kit in your pocket can have a cheap "bic" style lighter and a reliable firestarter. The backup kit in your backpack should have waterproof, strike anywhere matches and an even more reliable firestarter. It may seem a little obsessive to put each kit in a small waterproof pouch. So what! They're cheap, they keep the kit together and they give you one more layer of security.
By now, nearly everyone is familiar with multi-tools. It's pretty easy to find one to suit your personal preferences for blades and tools, isn't too bulky, and fits your budget. In my own experience, I've never needed big, beefy pliers, but I use scissors on just about every trip. I also prefer a good, comfortable knife that I can open with one hand to the blades in most multi-tools. So I opt for two tools rather than one: a lightweight, compact keychain multi-tool and a lightweight liner lock knife.
Most field repairs can be handled with only three items: a good needle (take two, they're small), thread (dental floss or medical suturing thread are incredibly strong) and duct tape! The thread and the tape can be wrapped around any handle that doesn't have a folding blade. The needles can be taped onto just about anything, like your water bottle, with a piece of the duct tape.
The rule of thumb is to carry one extra day's worth of food for each person. Calorie dense foods that don't spoil give you the most bang for your buck. Good choices are granola bars or energy bars, trail mix (or make your own), and meat jerky.
Smart: plan your trip so that there are safe water resources available at reasonable distances along the way, and refill at each location. Smarter: do that and take a water treatment system with you. Smartest: Make sure that your treatment system includes both filtration and purification capabilities.
If you have planned anything longer than a day trip, you are probably taking some sort of shelter with you. If unforeseen circumstances turn your day trip into an overnighter, or if something happens to your tent (stolen?), an emergency shelter, an ultralight poncho/tarp or even the shiny silver space blanket might be all that stands between you and a very cold, wet, sleepless night. A few ounces could save your life.