True, a normal healthy animal with access to water will generally not allow itself to become dehydrated, but there is a difference between being simply hydrated and being well hydrated. Probably one of the best things that we can do for our animals during any time of stress is to encourage them to drink as much as possible, not just the bare minimum that they need to keep from being dehydrated.
Regarding what molasses does for the goats, other than a flavor enhancer used to cover up different smelling and tasting water, it is valuable in helping prevent ketosis in does during the last weeks of pregnancy. Propylene glycol is the best preventative, and also a main component of the cure, but molasses is cheaper and easier to give. Sweet feeds may help, but I don't think that they are generally fed in large enough quantities to be as effective, and the sugar is more quickly and easily available to the bloodstream when dissolved in water than when given in the feed.
Although an oversimplification, ketosis is partially caused by low blood sugar, when the developing kids sap more of the nutrients from the doe than she needs for herself. Any sugar will help, but in the form of molasses it's more readily available and easier to administer. Nothing irks me more than this anthropomorphic mentality which leads people to misunderstand a valuable managemnt tool for something associated with a treat, like candy. I can asure you that professional nutritionists do not develop livestock feeds with molasses simply because they want to give the animals a treat. The dangers associated with this kind of thinking are evident here by the thought process which leads some into thinking molasses=sugar=sweet liquid=koolaid. As any sugar will help give the doe the boost needed to prevent ketosis, actual koolaid with a sugar base will probably work. The problem becomes that today we have a high number of koolaid like products on the market which contain artificial sweeteners which do not contain the vital sugars necessary in ketosis prevention. I would hate for someone to make the unfortunate association with molasses simply as a sweet treat, use a koolaid type product instead, with an artificial sweetener, and lose a doe or kids as a result.