To put the OP's argument in perspective, here are some real medical facts:
http://www.pponline.co.uk/encyc/0298.htm
For each per cent of body weight lost due to dehydration, your performance slips by about two per cent, and a meagre two-per cent loss in weight can force your heart rate and body temperature to spiral upward, making strenuous exercise almost impossible to carry out.
If you're going to be exercising for 20 minutes or less, dehydration is not usually a problem, but difficulties can arise during longer exertions. For example, copiously sweating athletes can flush about 1.5 Iitres of fluid per hour through their sweat glands, a total of three pounds per hour. If these heavily perspiring individuals weight 150 pounds, that's a two-per cent loss in weight after just one hour, producing a four-per cent dip in performance if no fluid is taken on board. The downturn in performance would be smaller, about two per cent, after 30 minutes, but that's still enough to make a difference to serious athletes who are interested in winning.
If the guy is just cruising along rather than racing (which I rather think is the case) then his power output and the amount he sweats are going to be much smaller than for a racer. Remember that the power needed to hold a speed rises with the CUBE of that speed, so cruising at 15mph takes around a fifth of the power output of racing at 25mph - that's a lot less sweat. No one is going to risk becoming dangerously dehydrated by a 20 mile ride at a moderate pace. And if the OP loses a couple of pounds of body weight in sweat and that causes a 2% loss in performance towards the end of his pleasure jaunt... well, is there any reason he should really care? Not everything is a race.