The most important part in here by SPJ is "haskell is a playground for language design" and by John that "haskell's strength is playing with DSLs". Haskell started and is a research language; that is PRO and not a con. It gives simon the ability to give a talk at POPL saying look at this cool dependently-typed feature, and btw, the compiler you just downloaded already compiles these programs. We need more languages that are bridging the gap between cutting edge research and production compilers.