Hi,
The Messages beta was released before Mountain Lion was Available is one answer.
There is probably some truth that people are "encouraged" to upgrade to new OSes and versions of Apps that work with them.
I certainly bought a new Intel computer so I could continue to help with iChat and now Messages (Note to self. Must get a life).
I can certainly appreaciate the annoyance at the push forward.
I have a G4/1Ghz tower that has extra drives and can run OS 9 and has partitions and Drives for Jaguar through Leopard which is about 5 different versions of the OS. My MacBook Pro which a Core Duo processor only manage 3 versions from new - Tiger, Leopard and Snow Leopard - it cannot run Lion or Mountain Lion which needs the Core 2 Duo Processor or later
Messages (Beta) does give you that ability to use the iMessage service to contact iPhone numbers and people using iMessages rather than AIM or Jabber.
FaceTime allows you to Video to those same Contacts.
I don't think it is likely to be a server issue.
In Messages it seems you cannot register your Apple ID if the Motherboard is not storing the Serial Number.
This can get removed during a repair or refurbishment (my understanding is that is it supposed to be replaced).
However this check only seems to have arisen with OS X 10.8.2 and maybe Messages beta is being removed as it does not make this check. (it would be one way to verify the Mac was a suitable machine or in fact a Mac).
10:54 PM Saturday; November 17, 2012
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously