I used plenty of leaves as a substrate component years ago, but stopped because they have a tendency to cause fungus gnat outbreaks even more readily than flake soil alone. Flake soil can certainly lead to fungus gnats too of course, but I found that it's only sometimes, while with leaves, there is usually a guarantee of an outbreak, eventually. I think that leaves might actually be more important in getting flower beetles to lay eggs, than for rearing the larvae. Leaves only seem to be of significance to promoting egg-laying in some species, however. They're definitely not needed to stimulate reproduction in thula and fulgida though, the larvae of which are very generalist feeders on plant detritus.