Consider it just mailbox-specific DKIM, there isn't a huge practical difference in reality. If you don't like hosted S/MIME then you could always find a provider that won't do it. At best it would indicate "it wasn't someone abusing the person's SMTP credentials" - a green checkmark behind a dropdown, not more.
For the vast majority it's a chance to introduce a tiny bit more verifiability to the ecosystem¹ but with the potential of actually making stronger implementations possible or even recommended.