I understand that carrying a few spares can be productive, but I don't think that simply replacing a supply is a great diagnostic for an intermittent because you'll need to wait for a while, possibly days, to see if the problem goes away. If the problem doesn't go away, you'll be back. Of course the power supply could be intermittent and pass a voltage and load test while you are watching, but this has not been my experience. My experience has been that the voltage is low, sometimes crossing the threshold to cause device trouble. It's a quick, easy, positive diagnostic. Some of the supplies will perk up a little if you interrupt their power. (the output voltage rises enough to operate the device, but the the voltage is still low) Without a voltage test this can result in a false conclusion that rebooting the device resolved the issue when the device was the victim, not the cause.