I took a platypus across Nevada and Utah, but once I hit Colorado, the services were close enough together and there were enough people around in case of a serious emergency.
Even then, I carried one or two 1-liter bottles with me in certain areas, just sticking them under a bungee.
I wouldn't attempt western express without a large water reserve, but I think that anything more than a liter or two is probably overkill for many parts of the US. As always, the region dictates the necessities.
As to whether a camelback bladder is easier to deal with than bottles, that's a personal choice.