That wouldn’t be a good practice, and honestly I am surprised that works!
Email addresses should be unique – 2 recipients should not have the same email address.
Manually adding an email address on the General tab of an AD object doesn’t really buy you anything, except the ability to expose that email address to processes/software that accesses and exposes the attribute. Those users are not mail-enabled (or mailbox-enabled), as you noted. If it’s not really required, I would remove the email address from the General tab of those users.