I have two Brooks saddles. The B17 on my tourer is certainly comfortable enough to ride for longish distances in street clothes. I have well over 10000 miles on that saddle and although well broken in, it has not stretched much, I've hardly had to resort to the tensioning screw at all.

that is also true of the Brooks Swift (with Ti rails) that I have on a road bike. Again a very comfortable saddle, but being a narrower racing style than the B17 I wouldn't spend all day on it without cycle shorts.

They last a long time. You don't want to get them soaking wet, though, so mudguards/fenders are important if you ride in rainy conditions. The small amount of moisture that gets at them from the top when riding is no problem. As for leaving them outside in the rain, I keep a shower cap under the saddle and slip it over it when off the bike.