The coctails are used for quickly rebuilding a new setup after tearing down an old one. Essentially it is a respecialization currency. But it only saves construction time, you still need the materials. It is the weakest major event currency I have seen so far. But ALL major events have topaz upgrades so it is not a bug, this game expects you to play it for years so you will buy the topaz upgrades bit by bit each year.