I think a big issue here is that previously if your car failed the MOT (prior to computerisation), you could take the car elsewhere, and it could either pass, or fail on even more items.
This is because whilst the level of competance amongst testers is generally the same, each tester may view the description of what would pass or fail in the testers handbook slightly differently.
By not allowing you to take the car for a test elsewhere after a fail (although this isn;t something that I had heard about), I suppose this stops you going to a more leniant tester when your car clearly shouldn;t have passed in the first place