I'd love to be allowed to assign a communication plan (that includes letters) to a student, even if we don't have a mailing address for them. Or vice versa - to assign a communication plan with emails to someone for whom we don't have an email address. We don't always have all of that information right away, but often want to start a communication plan as soon as possible (if not right away). It would be easy enough for me to copy and paste the letter text into an email, or vice versa, if needed.
Perhaps a pop-up window with a warning that you're about to assign a communication plan to someone with no mailing address/email address, rather than saying "you can't do that".