Q: What is the importance of this spec? Is the typical battery series resistance required?
A: The battery series resistance need not be the typical value. However, the max limit should be considered for the following reason:
If the internal resistance of the battery is greater than 0.5 ohm, there is a possibility that internal resistance (IR) drop is higher and due to this drop the effective VBAT voltage falls down under the low threshold for battery. In this case, the device will turn off and will not start charging.
This is particularly important with USB charging where the charging needs to be enabled by writing to register via I2C. If prior to writing the IR drops takes effect the device will not start charging.