MartinL:
But that's only relevant if the generic clock is being driven by a clock source that's active during standby.
For example in one application I used GCLK2 as run on standby, which is the GCLK used by the WDT on reset. Pointing it to another GCLK ensures that there is no unnecessary clock propagation.
Another example where this would be needed is if you have additional peripherals connected to the GCLK that runs on standby. Of course if you're running the CPU at high speeds, you might as well use another GCLK for them.
I'm not saying it is strictly required in every single application, but you have to verify that, and act accordingly.