Bikes are not made of spun sugar.
Brooks are not made of spun sugar, either.
They do not shriek like the Wicked Witch of the West upon contact with water, "I'm melting! Melting! Oh, heartless child!"
It rains in England. People there (where Brooks are made) have been riding them in the rain for over a century. No problem.
Mimi has a Brooks that is over 30 years old. She rides in Seattle. It rains.
I ride on my various Brooks in Seattle, too. For several years I've been riding my dear B67 in the rain for my commute. I've never had a cover for it. 23 miles in the rain, no ill effects. I raced in a tri with my soggy wet butt planted firmly on that B67, fresh from transition. No problem.
When the leather is dried out, rub in a generous dose of Proof-Hide on the top and the bottom, don't forget the edges.
No problem.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson