I admit I am a backup fanatic. That said, when I download firmware updates, I save them (not just for the phantom, for any hardware). I don't trust that the same version will always be available and I absolutely want a copy of what I have installed should I need it. Some manufacturers only keep the most recent versions, some only a few. Its rare that a manufacturer will have ALL past versions.
I don't keep it on the SD card of course. Why waste the space and the boot time. But I do keep it on a NAS at home and sometimes on DropBox.
Just so happens the FW I choose to run, isn't available from DJI any more AFAIK. Surprise huh? So, my backups paid off again. I have a couple new batteries coming tomorrow so I will be dusting off that backup BIN to make sure they match my current batteries.
Space is cheap. Backups are important.