The reason is:
"The backup battery is only 50mAh in capacity, intended to preserve the contents of memory in the two minutes of sleep time that succeed the third and last warning about low power in the main battery. It will continue to preserve the contents of memory for about a day, giving you plenty of time to replace the main battery with a charged one, or to connect the AC adapter."
This is a PB 1400 text, but the same backup battery is used.
So, one or two working main batteries would be "useful".
Maybe it's not worth the affort and it's easier to set the clock manually?
"The backup battery is only 50mAh in capacity, intended to preserve the contents of memory in the two minutes of sleep time that succeed the third and last warning about low power in the main battery. It will continue to preserve the contents of memory for about a day, giving you plenty of time to replace the main battery with a charged one, or to connect the AC adapter."
This is a PB 1400 text, but the same backup battery is used.
So, one or two working main batteries would be "useful".
Maybe it's not worth the affort and it's easier to set the clock manually?