Having separate batteries allows drivers to adjust the ergos for their preference on weight distribution, there's a mix of drivers I see who prefer racing with or without the base.
I've never looked at the charge status of both batteries but I know of many racers who only charge their NB4 once a month and will get 4 full race days on a single charge.
Just a guess, but maybe they want to keep the grip battery in storage mode and put all the stress on the base battery?
I...