How can not having enough voltage be harmful, other than from an unpredictable output?
Sounds unbelievable, but some devices REALLY CAN be damaged by too low a voltage. I'm not aware that PICs can suffer such damage but there really are some chips that can.
As Dippy says, at low voltages, the internals can get confused and that can cause internal 'shorts' which can destroy things.
PICs, (or any programmable device) can "forget" their program if programmed at a low voltage. This can happen within minutes, hours, days, months or even years after commissioning which can be very embarrassing.
Many switchers also suffer if input volts are too low. A good design will dissable things if the volts get too low but not all designs are good