Stock unit doesn't usually begin charging until at least 3,000 RPM (and usually closer to 4,000 RPM). Ours kicks in much earlier, so you're running off the battery for less time.
Additionally, the stock unit specs were something like 15V +/- .5V. This will pretty quickly cook a LI-ION battery and more hearty varieties of battery such as flooded cell will still benefit from a longer life span and lower maintenance interval.
I know I'm sort of blowing my own horn here, but for the money, it's the first upgrade I would consider on any compatible vintage bike.