Speaking as a 2013/2017 control staff member, I should remind you that with hundreds of riders using the controls, mains power sockets may not be available when you really need one.
[Also - speaking as a scatter-brained rider, and newbie GPS user - isn't this recharging* malarkey one more thing to remember when tired? I have enough trouble remembering my brevet card and bottles!
*whether from mains or battery-packs. ]