If you've treated the saddle then you needn't worry about riding it in all weathers, with or without fenders. When leaving the bike outside and rain is possible I simply cover the saddle with a shower cap, which I tuck under the saddle when riding so it's always available. But I do this more to make sure the saddle is dry when I get back on the bike than because I'm worried about durability. These saddles last and last.