Its my understanding that leaves are more an esthetic element. They look natural and are not difficult to maintain like moss. They provide cover for the frogs as well. That being said I have not tried to use them yet, perhaps in my next vivarium.
The leaves provide an alternative to moss for ground covering. It is cheaper(you can find oak leaves almost anywhere) plus if you use the leaves in your water when growing out your tads, it will provide tannins. Its like adding the blackwater extract from tetra to your water.
We were feeding some of our little ones the other night and I was looking into the terrarium when one of the magnolia leaves popped up like a hinged door! It was hilarious! We couldn't stop laughing. Peter Pan and the lost boys came to mind. Not sure why because I haven't seen Peter Pan in a really long time. The frog below the leaf realized that food was coming and came out to check things out.
We've noticed that the frogs tend to hunt more too (rather than just pick them off). The insects crawl over and under the leaves and the frogs dig around to find them! It's alot more fun to watch them eat. The magnolia leaves tend to last longer in a terrarium than the oak leaves.
We do like to add an oak leaf to each ventrimaculatus tad while we are raising them. We tend to have better success morphing them out that way, and considering we have 25+ out of the water right now I think it's helping a bunch. We've never added oak leaves to any of our other tads though. (We make our own tadpole tea that works well for us.)
Try it out. I think you'll like it. I know the frogs certainly do........