Ironically, just reading Ben Schneiderman’s (a tree map innovator) book on the benefits of collaboration. On aspect is how visualising data eases communication of otherwise unfamiliar data. Though not the intent of the original treemap concept, I can see a communicative upside in the ability to ‘gate’ the depth to which a treemap is visualised.
However, I think the hidden complexity here is that were that possible a single level-based ($OutlineDepth) cut-off would likely prove too coarse fro real world data where the sibling count (i.e. the point at which data proves hard to display in toto) will vary by outline branch. IOW, I suspect doing this right might be more complex engineering than assumed.
Thus, a very roundabout way of saying ‘plus one’ to the OP’s idea.