[noggin]

what is the purpose of usbpower option in boot.ini ?

See HERE (click)

This is the Primary reason why developer boards are sent out to various parties to pick up such issues.

The usbpower option has been removed (click) from boot.ini anyway with the Release Candidate - CE v9.2.4 - so there is NO chance of N2+ users causing damage.

Is there a functional reason why someone would need 5V USB port on suspend or off?
Is it to prevent some usb devices turning off?

You can use this setting to enable 5v power on the USB ports when entering suspend or power off,
# 0=off/1=on
# This setting will require a injected bl301 blob.

If a device supports 'Wake on USB' then having power to the USB ports is required? That might be a reason.