The new certificate is simply a print-out of the data held in a central computer. If your car ever goes through an ANPR check, the checks on insurance, VED and MoT are carried out automatically, so the bit of paper is nothing more than a reference. I suppose a forged one could be used to re-tax a car at a Post Office, but as soon as it went through to the DVLA, with no MoT in the central databank, the tax would be cancelled. As from 1 January 2012 the MoT test was heavily toughened up and now includes operation of ESP and airbags, among many other items.