I started doing that because I didn't want to carry spare batteries. If a tank is full, so's the battery, so I can grab it (and one or two more in the same state of fullness
) and go out the door without worrying. Since I primarily use one topper type (KFL2) and build them all the same, it's a pretty easy way to monitor cell condition.
If one is fairly consistent in one's vaping style, it does provide a rough estimate of actual battery condition. If, for example, a 2500 mAh one is still showing 1/3 capacity after a tank, and then after charging won't vape a tank, then the battery is shot. Once you start that type of routine, you'll get a feel for how it works for you.