I'm aware that you can do that, but it means that any other mod which has to change the ChunkProviders will conflict with it. Changing this would only be a minor change, but it would help compatibility.
I have been considering porting my mod, Cube World (http://www.planetminecraft.com/mod/cube-world-1227625/) over to forge, but it requires the WorldType to define which ChunkProvider a WorldProvider creates. Currently, a WorldType can only redefine the overworld's ChunkProvider, with the other two dimension's ChunkProvder's defined by the createChunkGenerator() method in each WorldProvider, but this prevents changing the ChunkProvider for those dimensions without modifying base files. There needs to be a hook in WorldType that allows it to define the ChunkProvider that each dimension creates.