Remember that friend who always makes the worst choices at races? My buddy Mark tried running a half marathon just going by thirst, and he totally bonked. He got dizzy around mile 10 and had to walk it in, which proves @evaramirez's point about the signal being broken when you're pushing hard.
Wait though, that thirst signal gets kinda broken during a marathon. When you're pushing that hard, you're already low on water by the time you actually feel thirsty.