At 18.63mph, the 208 mile ride is 11.16 hours of rolling time.
At 18.36mph, it's 11.33 hours. That's a 1.74% difference.
If you're racing, then it's a big deal. Even over the course of a 1200km rando, the difference between a SON28 dynohub and versus battery lights is less than 1 hour of rolling time. (235 pound rider, 25 pound bike, medium wide tires, 14mph average speed)
http://www.kreuzotter.de/english/espeed.htm
The difference for the same rider carrying enough batteries to stay lit for the duration of the ride (2x long-life Li-on packs) is a generated need for 2 more Watts of power to maintain the same speed. Less than the dynohub by more than half, so still the hands-down winner in overall efficiency.
Dynos are really a matter of convenience vs. price. Is it more cost effective to have 'on-demand' lighting which costs rolling efficiency, or to spend $500 or more dollars on a long-life Li-on (9 hour battery) light system which requires a charging stop?
To the 24 hour racer with a support crew, the batteries are probably a better idea. Maybe even for the credit card tourer who's staying in a hotel and can charge up each night. To the randonneur, who might get a few 30 minute naps at the roadside on a bus bench, the prospect of charging batteries might be slim. To the utility riders and commuters, the convenience probably outweighs the efficiency losses.