Could be semi-useful (Score 1)

As far as I can see, this would have some benefit against phishing e-mails provided mail from domains with no SPF entry was not regarded as spam/rejected. Then the choice of protection is left to the owner of the domain the e-mail comes from. yourbank.com can have one so they're e-mails can be identified as genuine. yourmailforwardingservice.com will not have one as it's unuseable. A little safe victory without losing any e-mail.