It doesn't apply to numbers, acronyms, logotypes, etc. There are several interesting issues with small caps. Use of small caps is a decision for the designer. It depends partly upon the font. In fonts with a small x height, you really want to use small caps.
Full caps are just too obtrusive.
In fonts with large x height, you may choose not to. Typographers like small caps. But outside of books about typography, I seldom see small caps used in the text. It's more common for titles. I always use them where they are available.