Praying for rain on Sh’mini Atzeret is all very well. But there are places that get too much rain.
Sitting in the sukkah and getting drenched is no fun. Crossing the street and being deluged before you reach the other side is no pleasure. Why not say, “When we need rain, that’s when we’ll pray for it?”
The better approach is, “Without rain the creation cannot function. It’s not just me and my street that I should be praying about, but the whole of God’s world.”