It's very rare that you need to disambiguate letters. My handwriting only disambiguates words of you accept contextual clues, and even then ...

Stylistic considerations, some of which ride on centuries of history, overriding information transfer is extremely common (that's not an excuse, more an observation). Just look at how this site [HN] uses gradual desaturation to convey downvotes, abysmal UX, but the designer clearly felt it was fit for purpose.

If your are typing a password manually, it's helpful if the font you're using makes it very clear when the character is 0 vs O, or l vs I vs 1