http://gregorycollins.net/posts/2011/06/11/announcing-hashtables
While Haskell people prefer to use immutable/persistent data structures in their programs most of the time, there are definitely instances in which you want a mutable hash table: no immutable data structure that I know of supports O(1) inserts and lookups, nor is there an immutable data structure that I know of which can match a good mutable hash table for space efficiency. These factors are very important in some problem domains, especially for things like machine learning on very large data sets.