During the winter months I've noticed a lot more riders with Camelbaks than I do during the summer months.
During the cooler months two water bottles will suffice for me. I use my Camelbak (and water bottles) during the hot months because I drink a lot more than during the cooler months. I also ride farther and longer in the warmer months.

Am I missing something?

Many Camelbaks contrain storage areas. Some people use them to store extra clothes/gear as they ride - add and take off.

I bought a CamelBak Rim Runner. When riding I bungee it to the rear rack, when hiking or climbing it seems to be bottomless when filling with stuff to take

I'm the same as you, less water in the winter. However I don't see many cyclists during the winter to see if they have camelbak or not.

I put hot chocolate in mine, maybe that's what they're doing too?

It's probably the extra gear thing.

I was thinking maybe they don't want their water to freeze, but if you live in Atlanta that shouldn't be a problem. Maybe they have some warm liquid and want to keep it warm.

Another idea....maybe they figure they have enough crap on already, so why not wear the Camelbak? Plus, Camelbaks can get a little hot in the summer, but that doesn't matter during the winter months.

Maybe they drop water bottles because they're clumsy in gloves.

Maybe you're mistaking extra clothing and hoods for Camelbaks.

I use a Camelbak all year round but in the winter, especially with an insulating hose, I don't have problems with the hose freezing. When I used water bottles, I would get 2 or three drinks, if I was lucky, and the bottle would be frozen. Colorado winters tend to be very dry so you need to drink nearly as much as during the summer.

Stuart Black

No, these are the small models and almost flat against the rider's back.

I got a Camelbak after my water bottle cage broke off 10 miles into a metric century. I didn't bother to replace it. I'm pretty new, and was not that comfortable grabbing for the water bottle anyway, and I only want a sip or two. I love my small Camelbak! I can grab a sip at any time, and not fuss with the bottle. It has small compartments too, for a gu and my house key.

I hardly drink during the winter too, but like today with it in the mid 30s I did have my water bottle and did drink a little, but it was too dang cold. I've often thought about getting a camel back too keep the water warmer - just too cheap to spend the money.

I use my camelbak all the time with water i use it more during the summer.