i dont think its a big problem. as long as you proofhide it or something similiar (i use nikwax, which i also use on my shoes/boots) you'll be relatively fine. of course, you can scrunch a bag in the rails if its gonna pour for hours and hours, but i think sometime life is for living and not for obsessing about covering everything you own with more things. i know plenty of people (including some mess'rs) who have put their brooks thru rain/snow everything and with no problems. your mileage will vary.