Carrying a drink is often not a key kit requirement regardless of whether its common sense or not. So you can perversely be running on sun blasted open fell for 3 to 5 hours carrying hats, gloves, waterproofs etc in your bum bag and be kit check perfect when you peg out with sun stroke, fainting and/or dehydration
Thats why I said that carrying waterproofs or not makes little difference to performance; the weight saving is negilible. Carrying or not carrying liquid though can make a big weight difference but of course wiley and/or local runners have drink support waiting for them around the route. This means that runners having to set out loaded with water are by default running at a big disadvantage. Having bases loaded around the race route is not cheating (in the same dastardly way as not carrying a waterproof

) but is miles miles more effective.