I didn't ask you why YOU think they shouldn't be considered and just dismissed right now.
I asked what authourity has said that it's totally illegal. Furthermore, said authourity must have really solid and indisputable reasoning for people to accept it (the reasoning you presented is not near enough, by the way).
The way equipment works is that gloves increase strength, but decrease defense.
Shoes increase speed, but decrease strength.
And Armour increases defense, but decreases speed.
Those are constant facts applicable to custom moves. It's also worth noting that the meters cap at +200:
Look at the Attack stat in this picture (which only effects the strength of A button attacks, not special moves). It is at +148.
Now look at the Speed stat. It's at +53. Add those together, and you would get +201, so if you took the green gauge graphic in Photoshop and used it to fill up the rest of the Attack gauge, you'd be just a pixel over filling the gauge, which means it must cap at 200+ (and after seeing how these stats function at that level in Smash Run demos, I know they are not completely ridiculous).
So you cannot make your stats that ridiculous. For example, if you use only that glove accessory in the picture and leave the other slots empty, your defense will be left at -50 despite having the extra attack. But even if you try to offset that with a defense piece (like in the picture), you will find your Speed in the negative zone now, and in the picture above, the Defense stat is barely improved from the vanilla 0+ stat. So you will have to now sacrifice a bunch of your power to get your speed to at least vanilla.
So there is no way this system will be amazingly unbalanced, it's looking pretty fool proof so far.
I could say the same for analysing frame data and hacking the game to analyse hitboxes, like what was done in Brawl and Melee.
But that's just me.