I'm interested in purchasing a much higher quality sleeping bag that I ever have in the past. I've always used synthetic bags around the $100 range, but would like to purchase a good 800-fill down bag. The local outfitter is telling me that if I want it to last I need to buy a $60 liner to protect the bag. I'm not really interested in spending the extra cash and don't want it to take up the extra space. I don't ever see much discussion around bag liners. Is this a waste of money, or does everyone use these as a given?