I would imagine usb ports on network devices exist in order to facilitate the transfer of images and other miscellaneous files related to the functioning of the device. I find it unlikely that the usb port would provide power to charge external devices.
I would generally advise to use the port for purposes that would not potentially render the system unstable.
I once had a critical, core router reboot into Rommon on me because of meddling with a usb port.
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