I carry 2-3 plastic water bottles. One 16 oz and one or two 32 oz. That way, I can lose or break a bottle and still have a way to transport water. A hiker with a dog came to my shelter on the Long Trail in Vermont, and the dog decided one of my water bottles was a fun chew toy. I'm glad I had a backup.
Plastic soda/water/gatorade bottles are free, lighter than Nalgene or aluminum bottles, and they are readily available and otherwise destined for a landfill. One argument I've heard for not reusing plastic soda bottles is that they release dioxins in your water. This is an urban legend. There are no dioxins in plastic soda bottles. That being said, a group of chemicals called phthalates are sometimes added to plastics to make them flexible and less brittle. These can leach into water if you heat up the plastic. I don't cook with them, so I'm not worried about this. (Reference)
Another reason I've heard, for not reusing plastic soda bottles, is that bacteria can develop under the cap, but these get cleaned regularly during my bleach treatment, so I'm not concerned about that. If you know of any other reasons I shouldn't reuse soda bottles, please let me know at grayson.ryan@gmail.com.
Nalgene and other hard plastic or aluminum bottles cannot be squeezed to fill my cook pot with the Squeeze Filter. Or to get all the air out to treat the cap and threads when chemically treating the water.
I've never been a fan of hydration bladders, since they are difficult to get in and out of my backpack, hard to see how much water you have left, and not easy to attach to the outside of any of my packs.