If everyone is a measure of each type and very few people are actually pure meso/ecto/endomorph, then clearly these classifications aren't very useful at all. I mean, let's look at some other methods of classifying people that actually do work. So sure there are people out there of multi-racial descent (I'm one of them), but the classifications are pretty clean cut when it comes to ethnicity for most of the population. You don't have over half the population being "of indeterminate ethnicity" or something like that. So I guess that's a more useful model for classifying people than bodytype.
Another one: Male and Female - those are two rather useful classifications, though there are people who are a mixture of each group... But they are rare, so this classification method would work for sorting at least 99% or some absurdly high percentage of the population.
Anyway, with the endo/meso/ecto classification method, at least 90% of the population is a mixture of each morphic type, and only 10% of the population actually fits into one of the models... It sounds pretty useless to me for classifying people. It won't even tell you who is predisposed to heart disease, blood disorders, osteoporosis or other disorders, as ethnicity or gender can tell you... because each person is going to be a mixture of endo/meso/ecto.
Heck, it won't even matter because the endo/meso/ecto classifications are static anyway. I'm sure most of us would be considered mesomorphs by common folk who just look at us, but how did we start out? Some of us started out as fat "endomorphs" while others like me were stick thin "ectomoprhs." Unlike gender or ethnicity, which do not change over time (yes, transsexuals, but they are still technically their birth gender on a chromosomal level, and yes, you might point to Michael Jackson for the ethnicity thing, but whatever...)
Hmm... where was my point? I lost it somewhere. Anyway, the whole point of classifying or categorizing people is so you can determine what most of them are like. Since the whole ecto/endo/meso thing actually applies to very few people, it's useless. You won't see it in the taxonomy books any day soon, I hope.