Like so much discussed here, it depends upon circumstances and the environment. In an arid environment, that 6.8 oz could be the difference between life and death, while in a more fluid endowed situation, 27 oz would be perfectly adequate. I cycle all the time with one or two - in coastal California that is more than sufficient.

But in desert conditions, you need far more capacity - a mere 27 oz of H2O is well nigh worthless in extreme heat. Years ago at the head of the South Rim trails at Grand Canyon, the NPS had a sign (I don't know if it is still there or not) that proclaimed "a canteen with less than a gallon of water is just a toy" or something similar. And that is a situation where you have dependable water sources available!

My time in the desert has taught me to pay attention to hydration. You must either carry it with you or find it along the way, but you can't live without it. I have retrieved the bodies of those who thought otherwise.....