I would like to specify that I am in favor of a Landorus ban. Now that I got that out of the way, I would like to mention a few things. Do not blindly follow Smogon. Why? Because, by that logic, Greninja should also be banned as it was in the Uber tier in ORAS OU and was only made more bearable in USUM OU thanks to new additions such as Toxapex, Tapu Fini, and whatnot. This is an important point because following Smogon entails banning Greninja and only unbanning it in the future. I would also like to remind all our Smogon lovers that Greninja was suspect tested before Landorus in ORAS OU. In fact, Greninja received more pro-ban votes than Landorus did. PRO was able to adapt to Greninja, which is still extremely strong on paper, yet somehow keeps underperforming. There are various reasons for this, such as the priority spam, but this is not a post about Greninja. I only brought it up to showcase why PRO is inherently different than Showdown and should abide by its own rules. "Smogon banned it, therefore it should be banned in PRO" is a moot point. Can you tell me why you think Landorus should be banned? There are certainly several pro-ban arguments that are applicable to PRO. However, I would like to tell you guys why it should not be banned and let you figure out why you so adamantly want it banned.
ABR gave some good anti-ban arguments during the Landorus suspect test. In PRO, Landorus has the potential to beat all of its checks, with the exception of Cresselia, but is unable to beat all of them at the same time. In this, Landorus is somewhat similar to Greninja and Manaphy. It does not have a 4MSS since it can still effectively do its job with the moves that it picks. What do you run on Landorus? There are so many sets! Earth Power is mandatory, but the other options are so varied: Sludge Wave, Focus Blast, Psychic, Hidden Power Ice, Knock Off, Rock Polish, Calm Mind, etc. On Showdown, everyone was able to use and abuse a perfect Landorus with HP Ice and 331 speed (which is above the critical 328-speed tier). How many people have a good Landorus with HP Ice? Without HP Ice, which is hard to get, Landorus is prone to losing to Gliscor, Landorus-T, and many more threats. In general, depending on its set, Landorus-T can be easily revenge killed (especially if it chooses to run Calm Mind over Rock Polish) or even checked by the likes of Skarmory, Tornadus-T, Gliscor, Chansey, Latios/Latias, Gyarados, Slowking, Cresselia, etc. Can it beat all of them (minus Cresselia)? Yes, of course, but it is unable to beat all of them at once! Scout before attempting to check Landorus with your Gliscor or Chansey. On top of that, it is not easy to get Landorus in for free with the way that U-turn and Volt Switch work in PRO. Due to how rarely used Landorus is, players might not necessarily always prepare for it. You also have to choose between Landorus and Landorus-T, since you are unable to use both. However, the lack of preparation for Landorus is not a pro-ban argument. Why? Because Landorus is not overcentralizing to an unhealthy extent; many of its checks are naturally used on several teams and will remain highly viable long after Landorus gets the chop.
Do you know what is actually quite overcentralizing in PRO at the moment? Manaphy. That is something you have to be specifically prepared for in order to avoid getting 6-0'd. Power Whip Ferrothorn, Unaware Clefable, Chansey, priority users, and faster threats are all increasingly common. Yet, Manaphy is able to beat all those threats with the right coverage: HP Fire for Ferrothorn, Rain Dance for Chansey, maybe Mystic Water and a Modest nature for most Unaware Clefable sets, barring perhaps the uncommon Calm Unaware set, etc. Somewhat similarly to Landorus, Manaphy is able to beat all its checks, but cannot beat them all at once.
Despite everything I just said, I am in favor of a Landorus ban. I have read and researched enough to make up my own opinion on the matter. Does it mean that I am unable to acknowledge arguments that do not align with what I think? No, absolutely not! We should not be too Manichean or too categorical. In Landorus' case, I believe that the pro-ban arguments far outweigh the anti-ban arguments. This is why it should be banned, in my opinion. But do all of you know what the valid pro-ban arguments are? Have you even seen enough of Landorus in PvP to make such a definitive pro-ban decision?