I don't know if that's actually traditionally true of fixed-width fonts. Courier, for example, is pretty bad, and widely used by people who aren't using a fixed-width font because they're programming.
Traditionally you'd use a lower-case ell for a numeral one, since there was no numeral one on your keyboard. Nor was there a separate zero. All of this "different characters for different uses" nonsense is for that newfangled electronic stuff.