One reason why you should not let people register other people
Let us suppose that you have a website that allows and in fact requires registration. Let us further suppose that you have decided to be clever and viral; you will helpfully let people register other people, so Jim can tell your website 'create an account for Bob, here's his email address'.
(This is subtly different than registration confirmation email, at least in practice. Arguably it is worse, since you aren't even trying to confirm that Bob is interested, you're setting up the account whether or not he is.)
Don't do this. No, really, no matter how much marketing tries to talk you into this.
First, this violates one of the principles of modern email. You only get to bother people who've asked you to bother them; you don't get to bother other people.
Second, if you allow this I can assure you that sooner or later someone is going to get clever (or forgetful, or helpful) and register a mailing list. This generally creates comedy, except not comedy for you and not really comedy for the mailing list. But it is a good way to leave an indelible impression on people.
And I really do mean indelible, because many mailing lists are archived all over the place; I'm sure that you can find any number of such 'registration' emails that have been sent to the Linux kernel mailing list alone. All of those are going to come up when someone else who gets your registration email starts doing Internet searches for you or for the phrases in your email, and their impression is going to be even more negative than it already was.