I've never encountered a situation in which the One File - One Module imposed convention was problematic. I also find it greatly helps when reading new Haskell code bases. I think it's generally a sign of good project design when there are lots and lots of small files, each with one data structure and a few associated functions.
For most of what I want, being able to introduce additional namespaces would be good enough. Making them modules as well could open the doors to some need techniques.
(I don't need multiple namespaces per file so much for the finished program, but it's handy when developing.)