Can't find any information about this out there...

The "door" or "flap" that controls whether you have recirculating cabin air or outside air feeding the hvac blower has a weird window in the middle of it. (see photos) The one on my car used to have some sort of fabric flap over this window, but it has since ripped. Does anyone know why that door was designed with the window and flap in it?

I basically never set the selector to recirculate cabin air, so I am a bit bummed to see this gaping hole in the selector door since it means I am always getting a mix of outdoor and cabin air. But maybe there is an advantage to that which I am unaware of. Do other cars have a set up where they are always getting some amount of cabin air mixed in with the outside air?

From what I can tell, the door is not a replaceable item, you have to do the entire Blower Sub-Assembly. Is there any reason I couldn't or shouldn't just seal up that window with a piece of plastic or some gorilla tape? The door should still function, it just won't have that window.

Thanks for any insights and help.