I like an on-board charger. I used mine all the time when I was riding my GPR-S to work. I would ride to work, charge up the bike for a few hours and be ready to go for a ride at lunchtime. I would return to work, plug it in and be ready to ride back home in the evening.
Now I ride my Zero to a County forest park 30 miles away, plug it in at an unused picnic shelter, go hiking for a few hours and ride home without worrying about saving electrons.
I think they are worth the extra few pounds of weight and book-sized space that they take up on a motorcycle.