Nested particles
Imagine you have a modelled branch of a tree with leaves. You plant it all over your naked tree and get a fancy tree. Now you want to make a forest out of that tree. Now it's not possible. Workaround is to make a tree object with branches without a particle system.
Conclusion: we need to be able to use object with particle system in another particle system.
2 Comments
it's possible : blender.org/manual/en/modeling/modifiers/simulate/particle_instance.html
Yes, it kind of is. But it has its downsides and a bit off the overall design