It's a great offer if the car owner is confident in the state of his/her car. This should always be the case.
But, the reality is that most people use the MOT to identify the minimum amount of work that will keep their car on the road and they get this amount of work done to pass the MOT.
Halford's offer relies on most people falling in the latter category. Kudos to them for coming up with an innovative way of increasing their customer base.