The Human body only has a short time then it comes to optimal performa time. You speed years to master a fighting style and can only use it effectively for few years and then age takes away your youth,speed,endurance and strength. If Shadow Clones existed and people could used them to learn faster it would save years of efficient uses of skills before old age steals them. People against using shadow clones are fools,for they also can be used to increase learning too,you can study multiple subjects at the same time. Multitasking thats all it is.