I use a bladder in a light trekking day pack for day rides, so long as my bike is not encumbered with panniers, etc.

In general, I drink a lot of water and stay well hydrated.

I don't find the extra weight on my back to be detrimental. Besides, for the early part of the ride, the water keeps my back cool.

I also don't require my water to remain cold all day; as long as I'm hydrated, I'm fine.

Using a small pack also allows me to carry incidentals securely, like keys, money, phone, etc.