Sincerely Bento project for avatars human mesh, not only useless, but also harmful. As you say the vast majority of the avatars are human.
A) FACE
Linden has entered the slide to the deformation of the face, and instead each animation of the face depends on the morphology of the face = is not appropriate any changes to the face by the costumers, even minimal, otherwise it will create the animation of the face of the "MONSTERS" . Also two IDENTICAL faces, but with a different construction of the mesh (vertices and edge positioned differently in space) will not move in the same way. Furthermore, since the assignment of the totally arbitrary weights, the same head mesh with assignment of different weights will move otherwise.
it follows:
1) who does Animations, will not make animations for mesh faces if not for a specific face and not the other.
2) users that would be deluding themselves to reproduce your face avatar of SL will be disappointed
3) no sane creator of mind you can afford the luxury of bringing discredit on a head mesh market that will not satisfy any of its customers expectations, but, rather, let them down with faciali animations that do not work, just move a slide.
This and much more (I can not make a treaty).
B) ELBOW
They inserted tails, wings, legs .... but the elbows and knees of human avatars that implode when they bend, we want to do something? some miserable little bone for this problem you could use it.
C) HANDS
Not being tied to a backward compatibility for the animation of the hands is absurd not to have created a correct rig and a standard mesh of the hand on which those who make animations, could be based to create animations. The position of the hand of the avatar of the base has fingers very Lg between them and not existing the metacarpal bones, when you go to rotate the fingers to push them up against each other remains a space between them. "No problem, you edit the skeleton", true, the fact remains that each one will have his skeleton and there is no universality to get decent animations.