My understanidng is that the Ext Osc is utilized when increased sleep clock accuracy (PPM) is needed. Recall that as part of the connection event, the master and slave exchange sleep clock accuracy (PPM) and the accuracy is used to determine the uncertainty window of the slave device for the duration of the connection event. This means that more accurate clocks can actually save power.
In addition, the module includes ability to wake from DEEP SLEEP after a configured time. Using the internal clock, it can wake anywhere from 64mS to ~36 Hrs. With external 32KHz xtal, 128mS - ~144 Hrs.