For an example of a modular character design, check out Jesse Sosa's excellent example http://oldwiki.polycount.com/SkankerzeroModularCharacterSystem (the images aren't up on the new wiki yet)
A couple things factor into this decision. 1. Is the character seen closeup, i.e. facial animation in cinematics. Then higher texel density for head and shoulders makes sense. 2. Do certain parts use alpha blending, while others do not? If there's hair on the head but no where else, then you don't want the whole rest of…