unsafePerformIO is not nearly as useful for introducing effects as one might think it would be. Because Haskell is lazy, any effects you might want would have to be grounded in the toplevel IO monad anyway.

To answer you directly, I've read code in various Haskell packages, everything from mime to snap, and AFAICT, it is typically used when you need to call a C library. Suppose you need to use OpenSSL's digest algorithms, for example. Because we technically don't know whether or not that library will have effects, we have to use unsafePerformIO.