Really bad benchmark? Inappropriate choice of benchmark?
I have a hard time seeing the value in a Mandelbrot benchmark. I absolutely don't have a problem with benchmarks that make Haskell look bad (e.g. directory traversal, regex), but I would like a benchmark that provides insight into the code you'd feasibly be writing in that language.
That's a really bad benchmark. The power of Haskell is not that it's fast for generating the Mandelbrot set, it's that it allows you to things you couldn't do in any other language while still being fast enough to write e.g. compilers (Idris) or web services (Yesod).