You are right of course, we could always just leave the plant alone and not prune or train it.... I mean look at mother nature, don't those plants get along fine?
Sure they do, but look at the plants in your or a neighbor's yard that is landscaped. Pruning and planning make it look beautiful. You can plant all the roses you want, if you don't prune and keep them in shape, they look like hell and produce fewer flowering buds.
To those that say leaves produce energy, I agree, but not to the point that I'll let my plants suffer. By pruning out the center of plants and keeping the undergrowth to a minimum, you promote air circulation, light penetration and new growth.
I'm not raising a plant to smoke it's leaves. If a leaf or two is blocking what would otherwise be a productive bud site, I'm cutting off the leaves. You can also look at lollypopping or SOG grows.... those plants do just fine without excessive leaf growth. This was the first place I've heard in over 40 years of living, not to cut off a leaf of a plant, and it is just silly.
I will say that you should not be cutting much off after week 3 of flowering. Most pruning and training should be done during the more forgiving vegetative stage.
Rather than get into the debate, or starting similar threads that only end in BS, try it yourself. Outdoor plants I wouldn't bother, as the sun will get where it needs to no matter what you do. Indoor growers with limited grow space should make the most of their space and utilize selective pruning and other training methods to increase the vigor and growth, thereby, the yield of their plants.
By removing upper fan leaves so lower ones receive light, you are not losing any energy. The leaves that are below the cutting will receive light and more than make up for the lost solar panel.
And again, I have to ask you "don't touch that leaf" people, what about cloning and mother plants? Do not mother plants give up many cuttings with leaves and stems and yet they recover with more vigor and new growth? Why isn't the plant dying or producing less if we cut off leaves? Simple answer is, mother nature is awesome. She doesn't need us to baby over her offerings. Just as a cloner can keep taking cuttings, a bud grower can remove fan leaves that block light to bud sights.
It's common sense, no matter what charts and pretty drawings they throw out at you. Think for yourself.