You do not need to send it to a bsp compiler. You can export it straight to .OBJ (or load in the .map directly if you write a parser and tackle the plane equations) i.e.
When you are done, you feed your brushes to the BSP compiler that took all the plane definitions that made up a brush, and spit out a quake level.
The outdoor areas in Doom 3 were made in an older version of Radiant and they are probably still better than the majority of levels created by indie developers using any other tool.
You could only create shapes that were legal in the games it was made to edit
Of course you can. It has brush, face, winding, vertex structs which can easily be used / modified for pretty much any task I would likely perform if I was using blender for making maps. The code is also relatively structured and flexible so anyone with knowledge in C++ can tweak it.
You cannot easily change that just because radiant is OSS.
Anyway, what would you suggest instead? A non-artist spends weeks learning something like 3DS Max or Maya? I dont know if this is some snobbery or what but if a developer wants to make a game without any experience in art, this old ID editor is still one of the easiest routes to take for artwork that is good enough for most indie games.
Anyways, this is off topic to the thread so I am going to stop here. I would just like to state that the tools used to create older AAA games (like Quake III) should certainly not be ignored by indie developers because they in most cases still provide greater functionality than required.