Then you haven't actually been reading anyone's posts (or at least, read and understood them). If you're going to be rude and ignore people's arguments then I'll be rude and call you out.
Literally nobody has used the justification that miis suck without their alternate specials to promote their legality. We've said that miis aren't broken or uncompetitive with their special moves so they should be allowed by default (this is the thought process we should use for all aspects of the ruleset). This is different from custom moves because custom moves have to be toggled on by the game, so when that toggle is off, anything to do with it is unimportant.
Imagine if custom moves weren't a thing, and only mii fighters could ever change their specials, would you still be against them being able to use them? Likely not, and that's how you should treat the situation. Customs on doesn't matter when we're talking about a custom's off ruleset, it might as well not exist.
I've always thought the "customs off ruleset" was a ruleset where customs are banned because they are considered broken, not one where we just turn the option in the top right corner to off (something that seems very arbitrary and, oh right, was just about never even discussed until mii's entered the discussion). So therefore any customs off ruleset must be based on some sort of reasoning as to why customs are banned. Here are the reasons that I have seen as arguments for customs being banned*:
1. DLC fighters only have 1 option for each special move: If this is a valid argument, Miis should only have 1 as well.
2. Allowing moves 2-3 for each special move is inherently bad due to making the game to special oriented, therefore making the game too defensive: If this is a valid argument, moves 2-3 of mii fighters should also be banned.
3. Many moves 2-3 are considered broken or overly-centralizing, so it is healthier for the game to only have moves 1. These complained about moves includes some moves 2-3 of mii fighters. If this is a valid argument, mii fighter moves 2-3 should be banned.
4. As a logistical issue in some tournaments, getting all special moves is not something that can be done reliably. If this is the case, Palutena's moves 2-3 should be allowed if Mii fighters are allowed.
5. Most moves 2-3 are intended only for casual play and are not properly balanced for competitive. Mii fighters were meant to be customizable, so their moves 2-3 should be allowed. If this is the case, Palutena's moves 2-3 should also be allowed.
6. Sakurai has determined only special moves 1 should be allowed for competitive as shown by him banning them for For Glory mode. If this is a valid case, all non-omega stages should be banned.
7. I am a TO whose player base hates moves 2-3 for most characters I believe that All options should be made available to the player if reasonable and possible. In this case only mii fighters will be available for moves 2-3 at the exclusion of other characters should the player base be okay with this, whatever the reason for them being okay with it may be.
*From this point on I'm not going to use the terminology of 'custom moves' because it's, for all intents and purposes, a strictly semantic term. I will instead use the words to describe things literally based on their effect in the game.
Now, I don't agree with any of the above arguments (except 7, but that's not really valid here since it would be in a per-tourny thing), but those are the ones I can think of for a no custom ruleset. Unless I'm missing something.