As many posts suggest, a hydration bag has many advantages and I won't repeat them. It took me a long time before trying one, but now, I cannot imagine hiking/walking/biking... without one !
But for sure, it also can fail !

That's why, when hiking, I have at least 3 different water containers...
- a Platypus hydration system (1.5 or 2 liters .. ?)
- a 1.5 liter plastic bottle of mineral water,
- and a metallic (alu) canteen

Why ?
- the Platypus bag doesn't hold enough water for several hours, so I need more ; and it can fail ;
- the plastic bottle is used to replenish the platypus ;
- the alu canteen is my last-ditch tank : should I fall and crash the fragile plastic bottle and Platypus bag in the rucksack, this more robust canteen should hopefully sustain the fall. If not, I don't even want to think about my own shape ... <img src="/images/graemlins/confused.gif" alt="" /> <img src="/images/graemlins/laugh.gif" alt="" />

Usually, when I reach camp (or preferably hotel .... <img src="/images/graemlins/grin.gif" alt="" />), the alu canteen has not been used.

As for a flask, in a pant's backpocket, its small size means it can only contain some Port wine : I like a sip before lunch !!!! <img src="/images/graemlins/grin.gif" alt="" /> <img src="/images/graemlins/cool.gif" alt="" />