Every winter ex there's always some brainiac who straps his fancy camelbak to his webbing and ends up humping a block of ice.
I wear the camelback underneath the cbt shirt, even during summer. Less things to get snagged on, the better. A little funny looking and awkward, esp. with body armour, but everything's a trade off.
Its a good place to keep the minor personal items (pair of socks, IMP meal). Keeps the webbing free for ammo and serialized kit only, so if you get wacked, buddy can easily grab your webbing and carry on the mission and leave you the personal kit, all in about 2 secs.
Of course, all this was before the days of the TV.........