What about water? Half-day or whole-day fasts without water? Assuming that dehydration would not be a problem, what are the pros and cons?
I'd definitely approach this with caution. I do think conventional wisdom overplays hydration, and sipping water all day is clearly not necessary (the analogy I have heard many times is that cavemen didn't carry water bottles). But I am ignorant to the benefits of going a day without water, unlike the well-documented benefits of food fasting, plus, I see no reason to disbelieve the medical establishment when they state how quickly you can die or suffer serious harm without water (as opposed to food which you can abstain from for long periods). If you really have to try it, I'd definitely observe yourself carefully and stop it if there were any ill effects. I also suspect doing it in the heat of the summer is probably not advisable!
I'm really not trying to sound sarcastic (I know sometimes text doesn't translate well versus actual speech), but dehydration would probably be the only "con"!
Water obviously has no nutritional value, but it's needed for digestion, proper absorption of nutrients (when you finally do eat after a fast) and to regulate your body temperature. I agree with CT in saying that hydration tends to be overhyped these days, but all the same, I would drink water during a fast -- even in small amounts.
I recently finished reading Fiber Menace by Konstantin Monastyrsky, and he basically shows that most people are drinking way too much water, which is hard on the kidneys and flushes potassium out of your body, causing constipation among many other problems. I've gingerly tried cutting back on my water consumption, with very good results. Also, reading this book has allowed me to not be worried at all about the low fiber content of Paleo diets. The book demonstrates that low fiber is a good thing. The web site is FiberMenace.com.