Hi,
iMessages account on the Mac requires the Serial Number to be flashed properly to the Main Board so it can be read by the system.
The app then reads this and creates a Auth token that sits in the Keychain.
The Serial Number can be missing on Repaired and Refurbished machines if the flashing of the Serial Number to a new board is not done.
There are some reports that on working (iMessages) computers that the Serial Number has "Disappeared" for some at the Mountain Lion to Mavericks and for others at the Mavericks to Yosemite Upgrades.
The Serial Number is used as part of the process to ensure it is you on your computer that is using the iMessages account.
8:42 PM Thursday; November 27, 2014
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
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