Lightweight backpacking or ultralight backpacking certainly enable you to travel more miles in a day. I can easily go farther in my running shoes, with my fifteen-pound pack on, than I could with heavy hiking boots and a forty-pound pack. That isn't all, though. Going light also means more comfort, more freedom and even more safety.
Disadvantages Of Traditional Backpacking
Less freedom. It isn't easy to take a side trip up that hill, just to see what's there. Do it without your pack, and you have to go back the same way to get your pack.
More tiring. It's clearly more tiring with a heavy pack, and you probably won't enjoy yourself as much when you are tired.
More Injuries. Blistered feet, sprained ankles, sore muscles, and back and knee problems are just some of the common consequences of too much weight on your back.
More hassle. Removing and putting on a heavy pack quickly becomes a chore. Some backpackers start to leave it on during rest stops, just so they don't have to deal with it.
Slower. Weight equals slower progress, meaning less access to wild places, since you can't go as far on a trip. It can also mean less time to for enjoyable activities, like a swim in a mountain lake, or a relaxing evening in camp.
Riskier. Injuries are more likely with heavy packs, but that's not the only risk. The inability to move as quickly when a storm is coming or when an emergency requires you to get to a road, can be a real problem. Then there is the possibility of bad decisions due to tiredness.
Lightweight Backpacking Advantages
Safety. Have all necessary items, but bring lighter versions. Lighten the load, and you're less likely to lose your balance and fall, or otherwise injure yourself. You can also respond faster to iffy situations. Safety is also a matter of knowledge and experience. Expert survivalists are safer with no shelter than a neophyte with the best tent. Know how to use you equipment properly, or to read the sky for comimg storms, and you can go lighter and safer.
Comfort. Does 18 pounds on your back instead of 50 sound nice? I completely stopped getting blisters when I started using running shoes instead of hiking boots. There's no sacrifice here. Eliminate twenty-five pounds of pack weight, and you can add back a heavier coat, or whatever you need to be comfortable.
Speed and freedom. When I walk past overloaded backpackers struggling up steep trails, and I have an eleven-pound pack on, I remember being in their place, and I know I am enjoying myself more now. Lightweight backpacking allows you to move faster, but notice I say "allows." No requirement, just the option, and that means more freedom.
Never yet have I met or heard about a person who tried lightweight backpacking, and then went back to a heavy load. This isn't to say it's for everyone. Some hiker's bad ankles may require heavy hiking boots, and some bad habits require a big pack to satisfy them. Still, even a backpacker who needs a pillow and big rectangular sleeping bag, can find these in lighter forms.
Try it. You'll experience more freedom, more comfort, more safety, more enjoyment and less suffering. The first time you are fifteen miles into the day, and you realize that you can easily run up that hill - just to see what is there, you'll know you made the right decision. You'll love lightweight backpacking.