It rlly makes life easier :3
That's the point. It would be OP. I agree with an enhanced cauldron to accept rainfall as source of water for Watermill (Yes, I never suggested it because I was too shy ^^). But it would need to be a very littlepower, and nonworking with something else than Tank for rainfall near the Watermill.
It would be something like max 1EU/t (on Storm, but 0.7 with simple Rainfall), but easier to set up than the actuals one (and working only when raining).
It would probably be denied because WM aren't for automated EU Generation.