Specifics are necessary because it can determine whether something is a fundamental property of a thing, how it influences the definition of a thing, how that definition influences the perception of a thing, and the relationships that thing has with other things.
A heuristic requires specifics to be defined, in order for it's definition to exist.
What we want the heuristic for is the average case that you dismissed, which covers the majority of us. There is indeed a thing as a large majority with mostly similar experiences and status, not just an collection of unique snowflakes.
PS: Sure, perhaps their not measuring 'happiness' and if it makes you feel better call it fruitkerfluffle if you want. However, that does not invalidate the question.