There is no valid reason for believing that rainwater will be clean
and fit to drink.

Well, all water we drink was rainwater at one time or another, wasn't
it? Of course, once it's collected and stored, there's possibility of
biotic growth, so it should be disinfected. But, if you open your
mouth skyward when it's raining, I suspect it's as clean a water as
you can get.

What completely nonsensical "reasoning". Suspect as much as you want.
But if you think rainwater "is as clean a water as you can get" you have
no knowledge of water quality - at all.

Quote from rainwater.com:

"Rainwater is a renewable, sustainable and a high quality water source
for your home or cottage.


Baloney.

Anf microbes are the least of the problems with rainwater. The
industrail and other human generated chemical pollutants are the real
problems. And depending on exactly where you are they can be very, very
significant.