Maybe some of us feel forced to use Avalugg to be able to keep Diggersby in check. And if you have a problem with Shed Shell on Avalugg, maybe run Knock Off instead of some weird mixed attacking set.While my reasoning is pretty much pro-ban Diggersby, I think Diggersby is necessary to keep Avalugg balanced, because of Shed Shell Avalugg. Shed Shell pretty much invalidate Goth and Wobb as hard stop for Avalugg, and the only good Knock Off user beside Diggersby is Mega Beedrill, Ambipom, and Mega Gallade (I doubt Ambipom and Mega Gallade is that good). If Diggersby is banned, you are pretty much forced to bring Kyurem-N or Melloetta to actually have a chance against Chanslugg core, or you have to waste a moveslot in your mixed attacker for Ice Beam, which has no coverage in Inverse. And if you think random mixed attackers can break through Avalugg... No.
4 SpA Dragonite Ice Beam vs. 248 HP / 252+ SpD Avalugg: 146-174 (37.1 - 44.2%) -- 99.9% chance to 3HKO after Leftovers recovery
Dragonite's Ice Beam won't 2HKO Specially Devensive Avalugg (which will probably be common if Diggersby is banned) and Avalugg can retaliate with Earthquake.
I have no good arguments to keep Serperior around and will look for one. I will vote later for Serperior.