I personally believe the "Default Mii fighters, 1111" rule is what we should go by. Why should Mii fighters be the only character with a custom move set? I understand that it is the point of the character but that is with the casual player in mind. Smash was never intended to be a competitive game.
Also to the point of "You should know your opponent." Would Mii players use the same excuse if all other characters were allowed customs? And what about DLC characters? None of which have customs.
Customs can be fun but, I feel have no place in the competitive scene.
This post is so wrong and that it was posted in a dead thread made me angry enough to almost post a long couple of paragraphs tearing apart your reasoning with gratuitous amounts of salty satire. But I'm better than that. I'll just show you why you're wrong.
The main premise behind your argument is that Miis have custom moves. This is
wrong. There is no debate. Custom moves are disabled when the custom switch is off. Miis still have whatever moves they want. In addition, Miis are given their own section separate from custom fighters in order to differentiate between the two, although evidently this did not work. I could be done here, but I'll entertain you a little more, just for completion's sake.
You also say you want Miis restricted because no other character can change their moves. Y'know, except Pit, Mario, and Marth. Or the semi clones. And the changes are less significant than switching to a full clone. One move is nothing. Four moves is getting somewhere, but this character still has the same normals, tilts, smashes, aerials, short hop, full hop, double jump, weight, gravity, falling speed, shield, forwards roll, backwards roll, spot dodge, air dodge, height, crouch, walking speed, running speed, air speed, air acceleration, traction, dash, and taunts. And some of those same things are actually groups.
Furthermore, players using a set other than the most optimal one is rare. Not only this, but that set is worse. Knowing one set of moves but not the other is like knowing the Mario matchup, but not the Doc one. You might be surprised by a thing or two, but, ultimately, Doc is bad in comparison and won't trouble you. Changing moves is no worse than counterpicking. If you have a problem with counterpicking, that's another debate. But even if this attribute was a bad thing, it still wouldn't warrant restriction. The same reasoning says that Shulk must stay in normal mode, Ryu must never use his strong attacks, Cloud must not break his limit, Bowser must not use tough guy, and Lucario must not use aura based moves (hint: all of his moves are aura based) just because these are unique attributes unavailable in any way to any other character. This is silly and should never happen.
To answer your question: personally, no, but that doesn't mean I shouldn't have to learn every move to be the best. "I don't wanna" isn't an argument against anything, much less an arbitrary rule in a competitive setting. And please don't try to deny that's what you're saying, because it is.
Don't be ignorant. Please. Learn the facts before you say something.