I can't dispute the instrumental value of a rational approach to building airplanes. But the instrumental value of a rational approach to religion is less clear to me.

It is difficult to estimate the instrumental value of anything when terminal values are unknown/unspecified. If one starts with absolute certainly that a particular religion is true, then rationality may have little instrumental value with regards to religion for that person (except to help them realize that absolute certainty is problematic). On the other hand, if one is deciding whether to join or leave a religion, then epistemic rationality would likely be extremely useful in making the right choice.