At the ashram when we'd do a 10 day fast, we'd spend the first 2-3 days easing into it, and then the last 2 days easing out. True, the middle 5 days were on water fasting.
The things that you say about fasting are true, but to be on H20 for a short term (say 5-10days) has some benefit. For one thing, all of that energy used to run the digestive system can be used for other bodily functions. Plus, since you're not really putting anything new into the body, the toxins are more likely to come out. This needs to be done under supervision, and when you are not working hard. It's the body's time to rest.
After the fast you go through a rebuilding period.