The main argument is that cold migration is no longer supported. Even if you manage to get a boot CD from somewhere (it can't be downloaded anymore), it won't support new features, i.e. it will most probably work fine with 2003 but hardly with 2008. If you have UEFI firmware and/or GPT disks, it will definitely not work.
OTOH, a situation when hot migration is not recommended is with domain controllers. You can search for KB articles on the subject if needed.
HTH
Plamen