From how I read Jello_Raptor comments on Announcing Mutation Testing in Haskell the library just surfaces these untested situations for you, it’s up to you to deal with them. The immediate goal here is to improve the test suite, which presumably will eventually improve the quality of the code[0]. I’ve done lots of minor “manual mutation testing” to ensure a new test covers a bugfix, so I do see the value.
[0] It has also been used in attempts to improve the quality of the coder.