I believe these parts are more for additional strength for people who close the doors by the extended glass, rather than for holding it from rattling in the door. It seems that when the door is completely assembled and adjusted properly, that the belt weatherstripping holds it fairly tight from the inside and outside at the top, the mechanism doing this from the bottom, and the downstop preventing the glass from ever going below the belt weatherstripping. These felts, in addition to the felted rubber bumper on the outside are always just supporting the middle of the glass. Additionally, these felt brackets can be adjusted fairly tight due to the thickness of the felt, at least when that thickness is still there, before it's worn down.

My gut feeling after having taken every part out of the door, putting every part back in, and adjusting it properly, is that the factory did a pretty good job of engineering the window mechanisms.