The more often it is used, the more of the goo is needed to get the same effects yet the recovery and exhaustion effects are greatly increased.
Additionally, after the initial exhaustion wears off, the goo could have an addictive nature that results in mored days of sweats and extreme weakness that lasts longer than the initial recovery. That would make the goo a dangerous thing to use unless they have enough quantities to keep those that use it going and are relatively sure of the safety for the two recovery periods.
I always like the subs that can be used as a plot point or simply to give the adventure more depth. If the locals see something they may attribute to a spirit they can invoke the name of Saint Duncan. Could leave it at that or if the PCs ask about it there is a few lines of history that they will get this info just broadened the scope of the game.
Cleric of the party is asked to exorcise a local demon/ghost in a villlage that has been haunting them. The cleric actually accomplishes it but the locals still "see" it around and see damage it continues to do. They are convinced that unless the cleric visits the church of Saint Duncan and uses his power and name that the cleric couldn't have possibly gotten rid of the spirit that plagues them.