Have unsecured messages be opt-in and have a warning banner on non-encrypted messages. Maybe even a confirmation dialog.
That way people who want or need to be that paranoid can be, but the rest of us can have something a bit more convenient.
By disallowing SMS messaging they've just made it so a lot of people who were being secure when their contacts allowed, aren't being secure at all.
I have a couple accounts I use as usernames to log into stuff, but I couldn't tell you if I've actually logged in to those email addresses in the last two years.