I spoke to the guys at one of the big companies about it a long time ago. I was told it was done to prevent air loss because the molding process was not always perfect and the cost of the tubing was less than having to check every single call that went out the door.
I'm not sure it adds anything and might even be not so hot an idea. The reason I called them is because the call I had was chewed up from being blown. Now it was a crow call, but the exact same tubing is used on the bite calls just a smaller diameter.
I would rather have it off. If I get air loss, that's what they make duct tape for

I think it would detract from your calls. They look pretty nice to be putting a piece of rubber over them.
Al @ THO