The silicone plugs look like a pour spout, and some people like to use them that way, though that wasn't our intention. We recently started sealing bottle tops with an induction sealer, after experiencing a few bottles with silicone plugs leak during transit.
The silicone plugs are helpful for sealing bottles after opening, if the contents aren't used all at once. I believe we plan to have the silicone nipples available separately, so people can purchase them to seal ink bottles after opening the foil seal, but I am checking to see the status of offering the plugs (I don't yet see them listed on our website to order).
Thanks for your input Brian!
Best regards and happy printing~ Dana