This would be helpful for SEO but also very helpful for reporting - because curently its difficult to report on where a form was filled in if its used in different places.
I can see huge value in being able to add a custom domain to a form! This customization would be very useful when it comes to sharing standalone form pages with clients. I suppose a workaround would be to add a HubSpot form to a HubSpot-hosted landing page so that it could sit on your own domain.