Danny asked me a good question about my previous post, which I'll answer here.
For NYCM, I've heard it's important to bring a water bottle to the start, because you'll be waiting for the start in the park a long time, and you may be standing on the bridge for a long while, and I don't think the water stops start until after the bridge (at 2+ miles into the race). By the time I arrive at the back of the pack, the supplies are often low!
I often carry a bottle in longer races to avoid the crowds at the water stations and in case the water stations run out of water. I'm not sure if it saves me time (because it does take a while to stop and refill the bottle compared to just grabbing a cup), but it does save me annoyance to just fill the bottle about every 2-3 stations and just jog through the others.
I don't like using a water-carrier pack as much because mine doesn't hold as much (sports gel, Kleenex, etc.) as my waistpack, plus they tend to bounce and chafe.
A hand-held water bottle allows me to keep hydrating more or less continually, instead of chugging water just at the aid stations. Keeping my water always on hand helps me pay close attention to it.
Plus I can monitor my intake a little more carefully if I pay attention to how many bottles I have drunk. Or if I think I'm getting a bit low on fluids, I can make a point of drinking half a bottle within the next mile, etc. I can also adjust my salt capsule intake more precisely to how much water I'm taking in.
I always throw the bottle away by about mile 22-23 (to save the weight at the end) because I won't be absorbing too much more water from my stomach after that to help me get to the finish line. (I use a cheap rinsed-out throwaway bottle for that reason, and not a fancy $14 handheld with a strap.) I can refuel and re-hydrate after the finish line.
It works fairly well for me in marathons, as long as I use a small, lightweight bottle. My shoulders just get a little sore if they're not accustomed to it.