As I understand the Cygnal internal oscillator, there is other reasons to make use of the internal (RC) clock.
Say, for example, that you have an application such as a data logger that is battery operated and want the battery to last as long as possible, and therefore, make use of the sleep feature of the uC. When you wake it up, the internal clock is usable almost mmediately. It is therefore possible to make some preliminary measuremments and determine if you even want to continue or go back to sleep. Since the X-Tal takes up to a millisecond to become stable, the internal clock could have completed as many as 20,000 instructions...thus saves power if you do not wish to continue.
Another idea which I have not yet been able to explore is the possibility of using the internal and X-Tal time bases to check each other in a sanity check. For instance if the X-Tal has hopped to a harmonic.
Maybe someone has had experience with this sort of thing and would be willing to share experiences.
TomB