I have only worn a belt for a marathon once. That was the year Chicago was super hot and I wanted to ensure I would have fluids. Even though there was a lot of press about them running out, I always found fluids and in hindsight did not need the belt. To be it is just extra weight. That said, it is really a personal choice and as long as you are training with it, no biggie

Oh, I guess you also need to consider what sports drink they have. If it is something that disagrees with you then of course you carry your own

Usually on training run, I drink 1.5 bottles of water (I have a friend meet me halfway with ice cubes that melt and give me some cold water for the torturous 2nd half). I'll be running my first full in October, and I'll probably bring along my water bottle because that's where I keep my chews. I also like to drink once a mile, not once every 2+, and I hate having to wade through the chaos of water stops. I can avoid the water stops when I'm feeling good, walk through them when I'm getting tired, and have water at my disposal throughout the race. If you're used to having water whenever you need it and have been training with the belt, then I'd say go for it, especially if it is going to be crowded and/or hot.

My hydration belt has two bottles on it. I don't mind carrying it since it doesn't weigh a lot and I do all my long run training runs with it, so I'm used to it. I'm also much better at drinking from a bottle than I am from a cup while I'm running. I'll generally drink water from my bottles and use the water from the aid stations to put either on my head, down my back or down my front.

Lots of variables, as everyone touched on. Temps, spacing, your anticipated finish time, etc. I would do everything possible to avoid wearing or carrying at a race, but sometimes it's just smart.

I am with Carriendar - I got stung yesterday on the rock n roll in Dublin ( it was a 1/2) it was really hot, stations every few miles I missed belt something serious!

I've only done 1 marathon (so far), it was a hot day (for Scotland), there was water stops every 3 miles, with gels being handed out from mile 13 according to the map (it changed to mile 9 for the gels), Edinburgh marathon is brilliant in that respect, the support from local residents of the towns you run through is amazing, they're all out with cups of water, orange segments, jelly babies etc to keep the runners going, some of them even have a hose out to mist people if they wanted it.
I have gone 12 miles without water so I'm comfortable that I could do that again, that being said, I'm from Scotland, we've been hot the last 4 weeks or so, but the majority of the year the heat isn't an issue (snow maybe, but not the heat)!!

I did have a small stretchy bum-bag that I used for carrying my gels, but I had trained to use half a gel every 3 miles from mile 9, so I only had to carry 5 gels at 40g each, carried an extra couple just in case and could pick them up round the course if needed, just didn't want to risk the course running out of gels).