I ride with a CamelBak while MTBing out of necessity. If I could get away with MTBing without it then I'd do it. Those things make my back hot. My MTB doesn't have much space for water bottles (can only fit a single small bottle and if I ride at night, my battery goes there) and either way I still wouldn't be able to carry enough water for the longer rides I like to do. On the road, it's a different story. I can fit two large bottles on the bike and I can refill them at convenience stores or from water fountains. Also, I like to have access to stuff in my pockets during a road ride. I'm unable to reach into my pockets if I'm wearing a pack. And lastly, I don't carry as much stuff with me while road-riding as I do MTBing.