I'm not sure if there's much I can say regarding Landorus-T that hasn't been said already, but I'll throw this out there: it seems like every pro-ban argument boils down to one of two reasons: 1) how often it's used, and 2) the variety of different roles it can fill on a team. Here are my thoughts on each of them:
1 - People have mentioned how Donphan is constantly used in RU despite being lousy in that tier, so I think it's safe to say usage doesn't necessarily reflect a Pokemon's viability. And there are other examples besides that. Remember back in Gen V Ubers when the ladder was riddled with players using crap like Charizard and Espeon? Granted, I doubt any of these has seen more than 50% usage in their respective tiers, but the point still stands - the usage a Pokemon gets is a very minimal indicator of how "broken" a Pokemon is. Even if Lando-T had 100% usage, it still wouldn't make a difference in terms of its viability or value to a team than if it had 10% usage instead.
2 - None of the sets Lando-T runs are particularly overpowered or require specific Pokemon to counter. With Naganadel, for example, you would need to run something like Weavile or Assault Vest Tyranitar just to avoid losing to it, and would only run them for the sake of beating Naganadel. With Lando-T, you have both versions of Greninja, Kyurem-B, Keldeo, Alolan Ninetales, and Rotom-W, to name a few. All of these are either OU or OU-viable Pokemon that fulfill other roles on the team and also happen to be able to beat most, if not all Lando-T sets. Hell, you could just slap HP Ice on Tapu Koko and beat it that way. So it's not like you have to run specific Pokemon or specific sets to beat it at all.
Also, am I the only one who thinks that if Lando-T were to be banned, it would only be a matter of time before a "ban Toxapex" bandwagon came along?