The drag is not that bad. It's so little that you can't feel it. But the power output is low. It's 3W (at 6V) nominally, not 6W. They could make dynamos that put out more, but then the drag would be intolerable. So they're great for powering lights, but that's because lights do great with 2 or 3 watts. But it's not enough or cost-justified for charging except in a very few cases. And even if it's good for charging, you should still have at least one spare external battery.

I run dynamo lights, and they work great for me. I run them day or night, so there is no charging available from the system.