Anatomy differences are indeed real, the problem is when you start linking metabolism with anatomy without being able to back it up.

We're making steady progress in genomics, in not the so far future (maybe in 20, 30 years?), we'll be able to access it, through some sort of GWAS (genome-wide association study) and epigenetics studies -- until then, it's just some arbitrary and often subjective classification, with different meanings for different people.

I do believe that it's fine to use those terms on casual conversations, the problem comes with the random information that, usually, comes with it and sometimes can hinder people's progress (see my other comment as to why i think that, under this parent comment).

For those unaware, the whole "body type" thing came straight off pseudoscience (somatotype and constitutional psychology).