You seem to be misunderstanding the issue. Let's say you work at
someplace.edu. You want to send mail from home. With the SPF-type
being discussed, your mail MUST come from someplace.edu's server.
If someplace.edu won't set up an SMTP AUTH relay, what do you do? Your
dialup account will let you use the dialup ISP's mail server... But
mail will get bounced because it's not something from someplace.edu.
Hence, if no SMTP AUTH relay, you're screwed.