i would classify them as a convenience item, like power windows on a car...lol they will multiply on their own, but not as quickly as if there were no frogs in the enclosure. without them, you'll just have a bit more maintenance. removing poo and such.

and no, you won't grow mold and fungii without them. you will definitely get fungus on any wood the first month or so thay the tank is set up. it will be white and fuzzy. this is normal and will go away on it's own, so don't freak out when you see it. you may even get some musbrooms from time to time. tbey won't hurt the frogs.