This is a question I have asked myself many times, I've never gone into it.
When you are designing a typeface conceived as all-uppercase, which would be the best way to handle the lowercase slots? I recall that in the early stages of digital typography it was not uncommon to find either fonts with duplicated uppercase letters in place of the lowercase ones, or with the lowercase glyph slots left blank.
Clearly the first solution is better from a technical viewpoint, but I was still wondering whether it would be good in technical terms, as it basically states that a form supposed to be lowercase is uppercase instead.
As an alternative, I have seen digital typefaces which use the lowercase slots to provide the most important alternatives for uppercase forms. This could be handy in a typeface like the one that I am designing, where I have more than a single alternative for some letters, and the more important could be mapped in the lowercase, but I am still unconvinced by this practice.
What do you think about the topic, both on a theoretic technical side and from your past personal experience?
Many thanks in advance for all the possible advice, it will be much appreciated.