Being able to shoot with one battery until it dies, and shoot while another battery is charging is invaluable. One battery and one is constantly worried about shooting or not, or deciding not to take a picture in favor of another down the road. Just one extra battery and these concerns fall out the window.
I usually shoot my batteries until they are exhausted, then charge them. I, of course, charge up before a trip or important day of shooting. I also charge up my batteries occasionally when I inventory/organize my gear.
In my travel kit, I keep 2 extras and the one in the camera. During the winter, I keep 3 extras, as cold temperatures impact lithium ion performance.
I keep dead batteries in a different compartment in my bag then charged ones. The same compartment as my charger, so I don’t forget to charge them.