I just thought it would be a useful and interesting addition. Would be great for example if you were creating a lot of trees on a landscape to have the brushes you are using to create them scale down into the distance. Or if you wanted to do it image hose style/stamp style even better. I forget, does TBP have an equivalent to an image hose? Would be great for those trees. Especially if it had some randomization options. i.e angle variation, scale variation, color tinting variation, scatter.... you know... the usual suspects.
Anyhow, veered a little there. PD howler has a scale brush based on gradient in it's swap image, but that's the closest thing I've seen outside of 3D software to my suggestion. It would be useful for a lot of things though.