Is there benefit to signing unencrypted emails? I came to this article from another comment on Crypto StackExchange. After reading it, I am wondering if there is any benefit to using a digital signature on unencrypted S/MIME emails?
Yes, of course there is a benefit to signing unencrypted emails. The article you cite is solely about the combination of signature and encryption; it doesn't directly say anything about signing unencrypted emails.

There is an important concern raised by the article which does apply to unencrypted emails, but that's because that concern applies equally whether the email is encrypted or not. The problem is that only the body of the email is signed. So when you receive a signed email (whether it's encrypted or not), you need to be careful not to implicitly assume anything about the context of the email. For example, when Alice signs “I love you” and sends it to Bob, Eve can forward the message to Charlie; Charlie will then see a signed message, guaranteed to have been written by Alice, and saying “I love you”. Charlie might assume that because he received the email, the “you” in the email is him; but this association is not warranted. All that Charlie can know is that Alice declared her love to someone, but not who that someone is. If Alice had wanted to send an unambiguous message, she should have signed “Dear Bob, I love you”.