Arguments for a ban on Knock Off
1.
It's good on everything that gets it. Unlike say, Scald, Knock Off is disruptive and viable on nearly everything that gets it. Even though Bisharp/Landorus recently left, Weavile, Crawdaunt, Mienshao, Machamp, Shuckle, Escavalier, Gligar, Empoleon, etc all run it. An important point of its vast distribution means that
it's more sensible to ban the move rather than its many abusers. Even Empoleon, a special attacker, has reason to run Knock Off. Much like Sticky Web or Spore (which are totally different in nature to Knock Off), Knock Off is just so good, but has a great distribution which means that
2.
Everything good gets it. An exaggeration sure, but unlike Sticky Web, pokemon aren't used
just for Knock Off. Weavile is a great late-game sweeper, Machamp is annoying as fuck, Crawdaunt has Adaptability, Empoleon has Defog/Stealth Rock -- basically, all the Pokemon that can abuse Knock Off are themselves abusable, meaning they aren't dependencies like Shuckle tends to be.
3.
It's very difficult to switch into. Somebody earlier made the comparison that Knock Off functions like Earthquake -- this is only true in terms of power, but 11 (around 20%) of UU is immune or resists ground, which is pretty significant and prevents any serious Earthquake spam. About half of that number resist Knock Off, and several, like Hawlucha, take half from it. The fact that steel types no longer resist dark is critical to this -- since would be survivors like Metagross and Jirachi are massacred or nearly OHKOd by the hit. Even Mienshao has a small, but possible, chance to be OHKOd by Crawdaunt's Knock Off -- again, sheer power is not an important part of the argument, but rather, that coupled with its secondary effect, few Pokemon can switch in with impunity and survive. The megas, having items that can't be knocked off, are key candidates, but all with the exception of M-Aggron take serious damage, and as Kingpoleon shows, M-Aggron can't take any possible moves from common Knock Off users.
252+ Atk Choice Band Adaptability Crawdaunt Crabhammer vs. 252 HP / 0 Def Mega Aggron: 162-192 (47 - 55.8%) -- 78.5% chance to 2HKO
252 Atk Life Orb Mienshao High Jump Kick vs. 252 HP / 0 Def Filter Mega Aggron: 193-228 (56.1 - 66.2%) -- guaranteed 2HKO
252 SpA Life Orb Tornadus Focus Blast vs. 252 HP / 4 SpD Filter Mega Aggron: 298-351 (86.6 - 102%) -- 50% chance to OHKO after Stealth Rock
All of these commonly run Knock Off while 2HKOing Mega-Aggron. All of these are extremely common. Trying to tank Knock Off doesn't really work. So no.
The fact that we are discussing Pokémon to take on
one move specifically shows how centralizing it truly is.
4.
Knock Off severely hampers playstyle diversity. Losing a Life Orb might not matter much for an offensive team in the long run (although losing a Choice Scarf can be critical), but for balanced and defensive teams, there's a greater dependency on items, especially when that item is not leftovers. Porygon2 and Gligar are both good defensive threats that become very difficult to use in an environment where Knock Off is so prevalent. One Knock Off counter I didn't mention is Granbull, who I used on a stall team specifically to take on Knock Offs -- however, this becomes much more difficult on stall teams where there is no Pokemon that can take both a hard special hit and Knock Off. Chansey, which isn't completely worthless after Knock Off, becomes much easier to take care of. Although I ran Granbull + M-Aggron to deal with nearly all Knock Off users, I still believe that Knock Off hampers stall as a whole, and unnecessarily shifts the game towards offense.
