Absolutely, I agree....
I don't run an extender, so my (XP) MCE box is in the living room. It is so convenient to be able to RDC into the box to do maintenance under a different user account while someone is still watching TV (I have the terminal services DLL version from XP RC2 or beta2...whatever it was, which allows multiple concurrent sessions just like terminal services on Windows Server 2003).
Even if they limited it to a maximum of 2 sessions, and only while one of the sessions is running full screen Media Center; that would still be OK with me. I can understand why MS doesn't allow concurrent sessions on a 'normal' desktop OS (to prevent licence sharing), but in a living room PC where one of the sessions is just running Media Center in a full screen, it's not really two people sharing one copy of Windows (in the traditional sense), is it?
That would certainly be something that increases the appeal of Ultimate.