Who really knows what foods agree with YOUR body?

I've been thinking some more about the blood group diet and I'm still curious how such a generalisation can be good for everyone. Whilst our blood group might be the same, our life and health experiences are not so how do those get taken into account?
I was speaking to a women a couple of days ago who was praising this diet as it suggested to her not to eat white bread and cows milk. These are actually, products that are commonly known not to be the best for us - white bread because the flour has been processed so much there is no goodness left in it and...well...surely any animal milk is designed to be drunk by the baby of that species and so cows milk surely is designed for drinking by baby cows? what do you think?

The other thing I've been pondering on is the diagnosis by a machine of what one should or shouldn't eat. Some vegetarian friends of mine went to see a local homeopath/naturapath and he used some machine on them which said they need to eat meat and fish. There was no testing of blood or other internal physical condition and so I'm wondering how too this works. How can a machine 'listen' to a body's needs better than the owner of the body?