A leaked file descriptor is a potential security hole. If you want your code to be secure - and in this age of internet-based applications built by plugging things together, that should always be the case - you want bugs from not dealing with an access issue to result in a permission denied error, not someone being able to read stuff they shouldn't.

So while we can't fix all the holes related to this issue or the larger issues related to forking a threaded program, changing the default to automatically close things will result in improving the security of haskell programs.