Hard to disagree with basic phsiology when it comes down to it
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I doubt that the average person who drinks extra water thinking it's going to halt the aging process or make muscles grow is going to hurt themselves, really. But there is a point I could make about drinking tons of water instead of other water containing beverages and that is there are some good things nutrient wise you are missing out on. As far as the body is concerned water is water.
I agree in general that your thirst will guide you. But I would like to caution that there are certain situations where drinking extra water as a precaution...although not extreme ridiculous amounts of it...may be a good idea. When your are sweating out a lot or working or playing in extreme heat it is good to sip on something whether you are particularly thirsty or not. I have seen and have been in situations where the effects of dehydration snuck up on people (including myself) before they were really aware of it.
Edit* I thought I should back that up with at least a little explanation other than personal observation. You'll hear lots of arguments as to when the thirst response kicks in and whether you are already "dehydrated" to some degree but since there is not really a good gauge of what is "normal" then it all becomes a lot of pointless speculation. In general you drink when you're thirsty and your fine. And since most everybody given a choice drinks much more than that plus takes in liquid through food...you get the point.
But the problem when your out in the hot sun our something is that if you are dumping enough fluid and becoming dehydrated quickly enough you may feel a little thirsty at first but the degree of thirst doesn't increase proportionately with the degree of dehydration. I.E. when you are twice as dehydrated you won't necessarily feel twice as thirsty. In certain situations, with strenous effort in extreme heat it is just easy to become dehydrated quickly enough while ignoring the thirst and as you become more dehydrated you're not exactly thinking as straight anymore, so on and so forth.
Anybody who's been engaged in serious work or play in dry or hot conditions (even cold dry conditions) knows it's pretty easy to override the thirst response when you're busy. And as you become more dehydrated your judgement decreases and you can get a little "demented" meaning that thirst isn't so reliable anymore. So it always pays to be careful about hydration. But most of the "hydrate" mania we get today is hype.