Corus - From your post, it sounds like you are familiar with Game Theory concepts. So I think you will know what I am talking about below.
"If vaccinating 1000 children resulted in 1 death and 2 disabilities, and not vaccinating those children resulted in 5 deaths and 10 disabilities, then the vaccine should be worth doing."
That is the state's point of view. Not only that, they also consider the costs involved - the cost of just a few disabilities is nothing compared to the cost of treating all cases of measles & mumps, including complications, and the economic cost of their parents' absence from work for two weeks. It is not surprising that the state advocates and pushes for the MMR.
However, the parents are separate 'players' than the state, with a different POV and priorities. To parents, the cost to the NHS of their sick child matters not one bit. What matters to a parent is the health of their child, not only now but going forward. The only reason a parent will take the risk of a vaccine for her child is because she believes the risk of having the disease is higher. When vaccine risk is perceived to be higher, disease risk is very low, or probability of catching the disease is very low (as with 'herd immunity') it is inevitable that more people will opt out of the vaccine.
Quite a few Game Theory studies have been done on this subject, like this one.