There are no real reasons that this is a bad idea
There is one reason I can think of off the top of my head. DTS shapes can only have a maximum of 9 collision meshes until TGE 1.4, and coincidentally, Blockland uses TGE 1.3. Although the engine has been heavily modified, I'm sure it's still susceptible to the collision mesh limit. This means that you would have to model stuff as whole bunch of pieces if you want to stick them together and have an accurate collision mesh.
With terrain and interiors gone in the next version, there probably won't be a mission editor anymore, and there might even be a completely different map format that is much simpler since all you need to really save is sky textures, fog color, and the map name. You're probably going to have to write up your own map format and make a script that creates the shape where you want it.
You are going to need a brick somewhere, because you can't build on just a dts shape. You'll get a floating brick error.
If you'd like to know more about why we don't make maps out of DTS shapes
right now, here is some light reading:
http://www.rustycode.com/tutorials/convex.html
