.com .net "require" only 1 nameserver (Verisign). PIR/Afilias require two (hence the need to fake or use a real one).
Please note also that my parent comment was with regards to the statement "the registrars require you to list two IP addresses" not why that may or may not have been required by the registry.
It's possible of course that even though .com .net only require one nameserver, a registrar might require two for some reason. It's also possible (but unlikely) that even though .org requires two the registrar could take care of this by putting in a dummy record if they wanted and only requiring one of the customer.
End users don't deal with the registry. They deal with the registrar.
