"I see way too many people lugging water bottles on runs and its not needed. A good read is "Waterlogged: The Serious Problem of Over-hydration in Endurance Sports""
While overhydrating is a serious problem, I think it is hard to make a valid blanket statement as to how much people should drink when running, especially trail running in the mountains. There are just too many variables to consider: Temperature, altitude, terrain, level of fitness, individual physiology, pace. To illustrate using extreme examples, Shepherd, Baxter, Sawmill, and Taboose Pass trails in the Eastern Sierra, are all in the 9-11 mile range, but they all gain ~6,000' starting at 4500-6200' and topping out at 11,400-12,300' or so. There are very, very few runners who could run them without drinking any water, due to a combination of the abovementioned factors. Even at lower elevations, temperature, terrain difficulties, level of fitness, and individual physiology will determine the need to drink. Another example from personal experience is the Wild Wild West Marathon held near Lone Pine, CA. Very few, if any, people go without water on that course due to temperature, elevation to a lesser degree, and difficult footing at times. Still, I have no doubt that many people do drink unnecessarily in less demanding conditions because they've heard it's the thing to do, sort of like watching the masses carrying their water bottles around with them when working out in gyms.