I think you mean roles there, but even then, it's not cut and dry, as different dps specs of the same class will prioritize stats differently (and therefore gem/reforge)...
Wowhead has them at 42%/42%/31% for vanq/prot/conq (just went off chest for 10N, assuming same for all pieces).
Vanq covers mage/dk/rogue/druid, for 4 classes, or 1+2+1+4=8 roles (13 specs)
Prot covers warrior/hunter/shaman/monk, for 4 classes, or 2+1+3+3=9 roles (12 specs)
Conq covers pally/priest/warlock, for 3 classes, or 3+2+1=6 roles. (9 specs)
Since the distribution is even for vanq and prot, and differing for conq, and yet the roles all vary: it's class based, not role (or even spec, or vanq would have higher than prot, since druids have a 4th spec).