The value proposal is what will make the user want to register. I have never seen a registration form that had its value proposal inside the registration form or on the same page as the registration form when the registration form is on a different page.
People must be persuaded to the registration form, they must be activated. You do it by showing your value proposition on the landing page. Increase the conversion rate.
People do not press the newsletter subscription button when they do not know they can get incredible offers and tell them when they are about to enter their email address is a bit late.
As for keeping the forms as simple as possible: everything comes down to privacy and speed. People do not want to spend a lot of time registering for something and, especially, do not want to enter data that they will doubt they can use. Do not request phone numbers when communication can be done mainly by email. Things like that.