There are rationally beneficial forms of partitioning using that same skill - such as the application of estimated beliefs in appropriate contexts. That suggests that partitioning is not anathema to rationality.

To my mind what is much more problematic is giving a free pass to particularly enshrined beliefs may have a contagion effect on other beliefs preventing you from properly evaluating them. In which case our partitioned theist may even have an advantage. At least Wednesday knows for sure some of her irrational beliefs. How many of us can say the same?