This is an interesting idea and regularly-surfaced use case.
My concern is always how to legally manage data and opt-in in this scenario.
If someone can enter another person’s email on a form—what is legal framework (in whichever country matters) where the form submitter provides details. There is no framework for someone opting someone else into our marketing email, SMS, etc. So except in the case of transactional messages where we can prove business relationship there wouldn’t be a way/reason to use that data for external comms.
Sure, someone can fill out a form “pretending” to be someone else, but if they check the “i agree” box, the current legal frameworks apply as 1st-party relationships.
That is not the case with “on-behalf” submissions. They would require MASSIVE workarounds and protections to prevent accidental violations of those “2nd-party” contacts.