I changed the setting to '1' and tested it with both Paypal and Stripe and neither one caused a registration button to show on the thank you page.
The registration link is dynamically created; based upon the IPN information returned by the payment gateway. If the buyer arrives at the Thank You page, before the IPN is processed, the link will not be available. That is why an email is also sent; and is the preferred method for delivering the registration completion link. Please read the last 6 messages on page 2 of this thread:
http://www.tipsandtricks-hq.com/forum/topic/showing-the-member-registration-completion-url-on-the-thank-you-page/page/2#post-70185
