For some users, col-12 doesn’t say anything. This is why I assumed that percentages would be much understandable for end-users.
Anyway, as a dev, you already know that 100% means col-12, 50% means col-6, etc. So there is no need to specify them with col-{nr}. As a backend developer, I don’t use bootstrap at all (but I used it a long time ago), and now I really don’t remember what col-{nr} is for 33%, or 20%, when I need them (usually once in a few months) I always open bootstrap’s documentation.