If you put a bunch of washing on the line to dry and it had gone through a cycle of getting dry and re-soaked by rain again about 5 times due to you being too lazy to get it in, would you rewash it in the machine or just wait until it got dry again and wear it?
Do as tilly2 dies, the acid and chemicals in the rain can react with skin to cause itching, rashes and there could even be microscopic parasites in the rainwater.
It was then, though, evian - people defacated in the streets then, water made you ill then due to no purification. That's why people drank wine and beer then - no such excuse these days.
I agree with maidup, my mum always said rainwater softened the clothes. I have done this many times, wet/dry, wet/dry and I have sensitive skin, never harmed me!!