Hi Dragan,
Thanks for the suggestion. Although that makes sense for geometry, a ForestSet works anywhere that Forest accepts a scene input (areas, surfaces, references etc) where probability doesn't apply. For me, the ability to control the probability for a whole set of objects is one of the benefits. For example, in our promo scene, I created 3 sets, one each for small, medium, and large buildings specifically so that I could easily adjust their ratios.
Cheers,
Paul