Pruning fruit trees is like woodworking... you can build a table using all manner of joinery and techniques. Mortise and tenon, sliding dovetails, halflap and bridle joint, and all kinds of stuff that takes all day. It all works, some techniques work better for structural integrity, some are austhetically more pleasing, some are faster and easier. Fruit trees are like shrubs, in that they can handle severe pruning and snap back the next year like a weed. But, they're subject to a lot of pests and problems. I prune mostly to open up the form for better light and airflow and to limit production. They will produce so much fruit once they get going, the weight of it will do more damage than any pruning you do. In the wild, this feeds the critters and insures more trees, but they get all broken down and don't live long. When you see a nice looking fruit tree in the wild, it's a variety that isn't suited for commercial production. If it's all broken down and gnarly looking, it's probably a variety that's now been hybridized for orchards.
I'd rather have a reasonable amount of nice fruit than bushels of barely usable fruit. Apples, pears and peaches I mostly concentrate on an open form and good structure to support the fruit. Plums are a different animal altogether. They're extremely prolific producers and only need the occasional pruning when you get stems that form very shallow crotch angles.
The easiest way to figure it out is to find an orchard and ask them if you can follow them around and observe while they prune. Then scale back their harsh methods a bit if you're not trying to mass produce fruit. As others have said, commercial fruit trees are usually replaced often and the high cost of labor means that a lot of them don't want the trees getting over a certain height and diameter. It looks brutal and ain't pretty. You don't need to do that if you have a small number of trees and aren't trying to make a living off of them.
