Well the 17 meter high waterfall in Burgers Bush(highest waterfall in the Netherlands) is very important for the humidity, but this doesn't count for many other waterfalls in enclosure/small rainforest houses.
I'm missing one important feature of these waterfalls; namely the fact they are often used for the circulation of air within the exhibit. And you see them very often, that's right, but I really like them at my home zoo ( emmen, netherlands) which has some magnificent ones by the way ( especially the one in the kodiak bear enclosure).