The end result is (pretty much always) polygons, and like tweaking the assembly code produced by your compiler, it's always useful to be able to make small changes on a per-poly basis - to remove invisible polys, to fix rendering artifacts, etc. Beyond that small tweaking, though, there's really no need for polygon processes in environment modelling; brush-based modelling is much, much faster.
Consider Photoshop. It's the most successful 2D image app in existence. Many of the others borrow elements from it. And what do I think are the two things that make it so great? Layers and filters.
Now consider 3DS Max. The really great thing in Max is the 'modifiers' system; you can just stack up different edits on a mesh, vary them independently of each other (for the most part), code up and add new modifiers to the system, and so on.
Am I the only person who thinks that the modifiers system sounds a little like Photoshop's filters?
Here's what I'm imagining. A 3D scene is composed of 'layers.' Each layer contains 'volumes' - brush-based solid objects. Left alone, these layers are simply composited and the resulting solids CSG'd to create a polymesh... but it's more interesting than that. You can assign an 'effect' to a volume. This will cause that effect to be applied to solids intersecting the volume on layers below the current one. It works using CSG to cut the affected area away from the rest, so you can apply an effect to *part* of a solid.
It's like selection rectangles, but persistant, more flexible, and in 3D. Don't forget that by setting up another layer on top of our 'effect' layer with *another* effect in it, the 'selection volume' itself can be distorted.
Furthermore the effects can expose other modelling paradigms. Say I want to do some intricate modelling on a particular part of the world; I can create a solid covering the part I want to work with, and drop an 'export' filter onto it. The section contained within the solid is then cut away from the world, CSG'd into a polymesh, and exported to a model format... linked to the main file, of course, so when you make changes to this seperate model file, they're reflected in the main one.