There could be times when you elect not to upgrade your Firmware.

1. What you have is working perfectly fine in your application. There are no known bugs and you don’t need any of the newer features offered by later firmware.
2. This one is a bit more subtle: Imagine your program is very large, and is pushing the limits of available program memory space. Because new firmware features take up more memory space, it is possible for a larger firmware to take away from available user program space. Therefore, a firmware upgrade could conceivable “break” your program. But fear not, you can always “downgrade” the firmware again to a version that has worked for you.