Water quality isn't based on how much decor you have in your tank. Unless your talking about driftwood and even then the presence of a lot of driftwood would simply change the ph not the ammonia or nitrate/nitrite levels. The water quality is determined by how much waste your fish releases and how much you feed him basically. Dead leaves in plants that aren't pruned properly also contribute to poor water quality...
The only advantage of not having decoration is that it makes it easier to clean the tank. But it also makes it boring and plain to me at least..
