I once bought a used bike already equipped with a Brooks B-17 saddle. It was broken in to someone else's sitter, but that has never been a problem. By now it probably fits my sitter, or our sitters are remarkably similar in geometry, anyway.

I expect some people return a leather saddle because the break in period is longer and more difficult than they thought.

I also think perspiration will do more damage to your saddle than will occasional rain. Keep it oiled and perhaps consider a cover to keep perspiration from getting at the leather.