I guess my problem with waiting for our bodies to tell us we need water is that we rarely feel thirsty until we're really, really in need of water. I believe there's a study in the JH Journal of Medicine, but I don't have time to find and link to it.
Besides that, as mentioned by others above, there are those who don't ever feel thirsty and those who feel hungry when they're really thirsty.
I'm someone who needs to drink water all the time. I carry it with me wherever I go. If I don't drink enough water (and that means more than the 8 recommended during the summer in my case), I get really dizzy and nauseous. I don't get thirsty very easily, though. And it really does help my skin--because I can tell the difference. I wouldn't say that drinking the 8 is necessary for everyone, but I would say that there might be less of a problem with obesity in the US if more people put down the sugary drinks and started drinking plain water. You can imbibe three times the amount of calories/fat with shakes/fruit juice/alcohol than you would eating an entire meal--plus you're not full at the end. If you make yourself drink 8 glasses of water, you're almost certainly not going to have nearly as much room for other, less beneficial, substances. That, as they say, is why I'm never going to contradict the 8 glass dictum. Nobody died from drinking too much water under normal circumstances.
