Haha! I've been waiting my whole life for someone to ask me for shrubbery so I could use that joke. It's gotta be a once in a lifetime thing right?
Anyway, here's my trouble with adding more sliders. Once I've made a decision to put in a slider for everything slidable, I think it starts to diminish the usability of the application to present an even more intimidating wall of sliders. In my mind, I just can't imagine why someone would want shrubland, and not savanna, or plains. In game terms they just seem like exactly the same thing to me, they are all the place where humans tend to end up.
Another issue that may come up is that these values do tend to change as DF changes, trying to exactly find where shrubbery begins and ends may only result in temporary control. The purpose of the sliders is to give you general control over the values curve to emphasize certain areas if you want, but trying to pinpoint this or eliminate that is likely to fail. For example, when I first released PWDF, I had the rainfall needed for the jungle trees pinpointed, but now it's only a very rough estimate. It's still a useful slider since it exists at the end of the rain spectrum, but its no longer aptly named.
PWDF is already kind of a beast for new DF players, I really don't want to make it beastlier.