This statement is incorrect. According to the provided link, "Once the battery gets too low on the smartwatch, the gadget goes into a Power Reserve mode that lets you see the time for up to 72 hours from the moment its activated thats good for checking the time each hour for four times, with each check lasting 4 seconds."
So if I wish to do simple things such as time a patient's pulse, respiratory rate, and other similar timing functions (which take approximately 15-60 seconds per patient) that my dumbwatch would allow me to do, I would not be able to do this with the Apple Watch even in Power Reserve mode. Even using it as a stopwatch or countdown would likely be out of the question based on the short intervals the Power Reserve Mode would allow.
Link:
http://bgr.com/2015/03/10/apple-watch-battery-power-reserve/
Again, this is not a complaint. I am still very excited to purchase one for social usage, but simply responding to a poster's original question on what situations would require more than 18 hours. I definitely look forward to greater battery life in the future, but will happy to try one out for what it is as a first generation product.