I do not see how allowing Miis to have their custom moves available and not the rest of the characters to be going down a slippery road. Customs On or Customs Off IMO is the wrong word to use, the ruleset should be based on toggling the "Customization" button on the select screen on or off. Turning it off prevents normal characters from accessing their custom moves, but does NOT restrict the ability for Miis to use their customizable moves, as they are already pre-made sets that still can be accessed even with customization set to off.
Also, Sakurai rarely balances custom moves not named Dragon Rush and Burning Spin Dash, but he has gone all the way to balance many of the custom moves the Miis have this recent patch. It just feels like the game intended to let the Miis be free to do whatever they want, so I don't see why we should go against that. Heck, even places in Japan, the most conservative Smashers in the world, are allowing the Miis to use their "jank" customs.
I fully support Miis having access to all their custom moves. However, I personally still am torn of what size the Miis should be allowed to use. That might be saved for another discussion.