The thing with Blender as a map editor is that it would also need to be able to place entities, triggers, lights etc.
I cooperated a little with guy named Z80 for a map plugin for blender he made (I created a little of documentation) and it exports entities and stuff (you just need to add a property to every mesh you want become an entity). But the problem is that: is for Blender 2.49 ,it's untested and it's EXTREMELY slow.
Also, Blender has no grid (edit: I mean no automatic, mandatory grid that conforms to Quake units and can be switched by hotkeys). A grid is hugely helpful when building map geometry. The more complex it gets, the more likely you are to have leaks when you export from Blender to map. There's a reason they used Radiant for idtech4 games and not 100% Max or Maya.
I agree with you at 50%. The generation of game production until 2005 was as you described. Infact if you compare a Painkiller map (Great game guys, Great game), realised in 2004, 100% made in maya, with a doom3 map, the id map detailing quality is quite embarassing. There's a level made in a ruined monastery positioned on a cliff...magic.
2012 big companies games are trying to use a lot graphic software to make maps and then use the editor as container (just take a look at sample maps in UDK).
In my opinion level editor, for standard games nowadays, is barely useless, but of course is just my opinion.
Yes, of course, maybe you don't have the precision of grid, but you could have the possibilty to create an "organic" level.
Keep in mind the .map exporter is not an official plugin, so it wouldnt be the blender devs you'd need to talk to.
Are you sure? Because in a clean Blender 2.59 installation the plugin was in the core plugin library