Using open meshes also has the advantage of not having to render unseen polygons which adds to a faster frame rate. Take the case of holding a gun. You can't see the bottom of it, so why bother building faces and using extra vertices in that area when you could avoid them and speed up the rendering (or add extra details to what is visible)? If you can't see the bottom of a table, why bother wasting polygons and vertices to render it?