Dedicated h20 on the second line?

[BadMuther]

Most gear whores are always attempting to perfect their rig or see what new gear is out there. One deciding factor for me in gear wear/placement, is water. I know some folks always carry canteens/platypuses on their second line, and then there's those who never do, relying on a camelback instead.

I'd like to hear pros and cons on whether or not you think water should always be carried in the second line in something like a canteen or a platypus or whether a seperate camelback will do.

Thanks.