from my elementary understanding, a unbranded phone can use f/w updates from any carrier (eg. rogers most recent is 020, however unbranded can be upgraded to 026).
Rooting a phone simply gives the owner superuser privileges, meaning that one can have ultimate control of the phone....generally people root so they can install specific apps that do not work on unrooted phones (eg. titanium backup, barnacle tether, always flash, setcpu, adfree...etc etc)
so to answer your question, root is not the same as branding, HOWEVER, there are ways to unbrand and root and upgrade in one go