Battery packs are a better solution for almost all cases. Four of them weigh less than a dynamo hub. A bottle dynamo makes a lot of drag for a little amount of power. The charging circuitry is expensive. I think it makes sense to use a dynamo for charging ONLY if you're going to be away from a power outlet for several days.

I use a dynamo hub to power my headlight day and night. I find it to be cost and trouble and drag justified. I use battery packs for charging my phone, and I could use it to charge my taillight etc.