Hostility towards the program terragen has always been present, and most likely, always will be. But let's get down to brass tacks. What is terragen? It's a 3d scenery generator. Right. There's no real Modelling process involved per se, and it looks and works completely different (to all means and purposes of the majority) to a 3d modelling application such as 3d studio max or Maya. It is comprised of a series of mostly numerical controls, and a few random generators based on numerical/slider inputs.
...this means, it's an easy program, and requires little or no effort to pull off good results.
Here's my favourite word of this article