I think the optifast shakes came either pre mixed, or as a powder with dried milk as part of the contents, so needed to be mixed with water, and so will give a very repeatable balance of nutrients and calories. 'Cheaper' shakes mix with milk, but allow for the extra calories the milk will provide, but it depends on the customer using the correct amount of the right type of milk, but over the eight weeks the up's and down's will balance.
I suspect the Newcastle Diet would suffer if you drank extra milk though, as it will add more calories to the day.