I have wished (multiple times) that my children's prescription medicine had just such a plug. Their liquid acetaminophen does have a plug and it is FANTASTIC. I get to accurately fill the syringe straight from the bottle. Hallelujah! (And I can flip the bottle upside down and even flick the syringe in a very important looking fashion making me the epitome of medical coolness.)
You can imagine my surprise when White Coat Dad was cleaning up our acetaminophen bottle and said something about "does it matter if I popped this thing out?" The heavens parted and I beheld a miracle. The plugs pop out! I can simply pop a plug out of an old bottle, clean it, and save it for the next antibiotic round. Check, check, check and I am ready to go!