how much is this cost ? It looks a little bulky, wouldn't be easier to by 3 or 4 more batteries ?
You'd still need a multi charger though, otherwise you'd need to charge each battery one at a time. Not very practical.
This does the job of multi charging and effectively gives you 6 extra batteries when out in the field (assuming you have at least 3 batteries).
So with this and 3 batteries, you have a multi charger plus 9 batteries.

Also way cheaper than buying 9 batteries and a multi charger !