There is no harm in following a non-member. You're absolutely right that a non-member can still help your article get out there.

It's a matter of opportunity cost. A non-member reading your article is definitely better than nothing. But a member reading your article is often even better.

With Medium's following limit every day, someone who wants the best and fastest returns on Medium will have better luck following members.

That's not to say that following a non-member won't do you any good. There are many cases where success on Medium can be attributed to non-members, but at the end of the day, income from the Medium Partner Program relies on Medium members reading your article. A follower base consisting of mainly Medium members will generate more revenue than one consisting of mainly non-members.

Let me know if that helps!