Well the idea isn't that you are going to slavishly stay to these proportions. These are just the proportions for an average person for the various age and sex. A good artist will know when and how to break the 'rules' and still have something that looks natural and pleasant to look at, or if the case may be, ugly and unnatural. Learning these proportions is important. Building a model or drawing a picture sticking to those proportions won't leave your work looking rigid. Drawing or sculpting your work in a rigid pose will make them look rigid. Now if you keep to these proportions for all your work and don't vary the proportions any, everything will start looking the same and generic. Perhaps that is what you meant.
As for learning proportions and not measuring body parts, measuring body parts is part of learning proportions. Developing a framework for what looks natural, which is based on measurements, is part of the process. After a while it does become second nature. You won't have to think about the rules and measurements and you will just know what looks natural.
You are right though, this kind of thing is typically just for beginners who don't understand or know proportions. An experienced artist won't rely on this.