There are two things I can say about this, 1. the reason 1rst person armor meshes are done the way they are is indeed clipping but also resource management, less poly's gives more available resources the system can use for other things that will render in the viewable space. 2. 1rst person weapon meshes are used for the inventory preview window.
I do not think polycount makes much sense. Unless there are really machines where 3rd person view is unplayably slow?
I think it's
1. so that they did not have to worry about triangles from the third person mesh obscuring the player's view, and
2. so that they could use different animations (or at least different skeletons) in 1st person. (Though if it's only different skeletons it still doesn't make a lot of sense.)
In particular, notice how your character will hold weapons low in third person view and in front of your face in first person view. This solves the "which weapon am I holding" problem, for first person view.