Yeah, I think this attitude is just a side-effect of being new to FP and wanting to cling to familiar techniques.
I certainly felt the same way getting started. Then once you read the Out of the Tarpit paper you get over it and start to trust referential transparency.
I read the abstract and a lot of feelings I've had about testing in FP coalesced: I knew this intuitively but wasn't able to put it into words. Codd can be your pair-testing buddy for a LOT of FP testing.
I certainly felt the same way getting started. Then once you read the Out of the Tarpit paper you get over it and start to trust referential transparency.