Although I'm probably going to repeat some arguments that other people have brought up, I'm going to state my opinion on Greninja, and wether I think the metagame would benefit from having it gone or not. Greninja has almost unmatched capabilities as an offensive Pokemon, to the point where all playstyles have to prepare especially well for it, in order to stand a living chance against it. Its high Base Speed, good offensive stats, amazing ability, and great coverage, greatly limit the amount of Pokemon capable of switching into it. Handling Greninja defensively without sacrificing multiple team slots or running sub-par Pokemon such as Porygon-2 or Tentacruel is close to impossible, as you have prepare for all the options Greninja is capable of running. Handling Greninja offensively isn't exactly easy, either, despite it having poor bulk; its Speed forces offensive teams to rely on Choice Scarf users or priority to handle it, which is generally a liability, as being locked into the wrong move can easily be taken advantage of. No other Pokemon in the metagame is currently close to having all these combined attributes to the same effect of Greninja, and I believe that its presence is holding back the evolution of the metagame. And, while Greninja has its perhaps greatest matchup when facing balanced teams, I am willing to state that all team archetypes suffer noticably from Greninja's presence.
I've been hearing a lot of people say "Greninja needs to be banned, it is so anti-offense". I don't completely disagree with them, but I'd rather say Greninja is anti-balance. Offensive teams can afford running multiple ways of dealing with (revengekilling) Greninja without creating redundancy, becuase of the offfensive team structure. Greninja is still incredibly annoying to deal with, don't get me wrong, but offensive teams are one of the team archetypes that fare /well/ against it. Balanced teams, however, have a structure that simply doesn't have enough space to run solid answers to Greninja, as they are generally lacking options otherwise. Running multiple Choice Scarf or multiple defensive checks to one Pokemon is just straight up impossible on balance, which is why Greninja is such a nuisance. Nothing is capable of switching into all the variants of Greninja, which forces the team playing against it to scout for every option. Say, your Ferrothorn has switched into a Ice Beam and lost 40% of its HP. What do you do now? Do you switch out in fear of HP Fire or Low Kick, or do you risk staying in, banking on the opponent not running one of these options? The best option is probably switching out, as the probability that one of them are run is high. The Greninja user loses little to nothing from this; Greninja just gets of damage off on the opposing team, losing only what recoil Life Orb causes. The standard set of Ice Beam | Dark Pulse | Gunk Shot | Low Kick has coverage for it vast majority of the metagame, and has close to no viable switch-ins, especially not on balanced teams. The power in Greninja's ability and coverage also gives it the ability to pressure stall teams, just by being in the tier. Surprisingly enough, with a much bigger pool of Pokemon to choose from, there are very few Pokemon capable of taking it on. Some of the options are Specially Defensive Mega Sableye, which is, in my opinion, a waste of a great Pokemon. Having this liability which is Greninja is currently limiting the freedom in teambuiling, which ultimately means the evolution of the metagame. We don't want the metagame to settle, stopping its development, like the case was with Aegislash.
Some of the arguments against the banning of Greninja that have been stated in this thread are the following: "Greninja is very frail, so it isn't hard to revengekill", and "Greninja is OHKO'd by Conkeldurr's Mach Punch, and it cant OHKO back". The thing is, how do you get your revengekiller in? You have to sack a Pokemon, or let your revengekiller get severely weakened. None of these are profitable for your team. Also, mind you, Greninja can always switch out when faced against something capable of revengekilling it, which means that the next time Greninja comes in, you have to sack another Pokemon. Thanks to its Speed, Greninja's frailty doesn't really count in for it. "Greninja has 4MSS". This is another misconception; having a lot of viable moveslot options isn't neceassarily the same as having 4MSS. Four Move-Slot Syndrome is when a Pokemon has more mandatory moves than it has move slots. This is not Greninja's case; it just has multiple viable options, of which lets it beat different defensive checks. This is not a hinderance at all. All in all, I think the metagame would benefit from getting rid of Greninja. Its amazing combination of traits lets it pressure all playstyles, some more than others. Dealing with it is incredibly hard, being forced to run Choice Scarf or priority users is putting the metagame's evolution at halt. I'd like to see Greninja BANNED.