This has came up a lot so I just want to say that I think this argument is null and void. If banning Ferrothorn means Latios becomes broken, then we ban Latios too. If Ferrothorn is indeed the only thing keeping Latios and company in OU, then it's probably broken. Look at it in reverse - lets say (hypothetically) that Garchomp is broken. Should we bring down Lugia to keep Garchomp in check? Surely not. We shouldn't bend over backwards to keep a broken Pokemon in OU just because he happens to stop another broken Pokemon.
I just want to say how very much I agree with you here. Absolutely spot on.
I could write a goddamn argument on the centralisation around Ferrothorn. The most notable point is the death of the Bulky Water - a staple in nigh on every decent team for generations. Wanna explain to me why even the mighty Suicune is UU now, Swampert is a laughing stock, and the few that still see use are forced to run Hidden Power Fire (Rotom-W) or Taunt (Jellicent)? How about the fact that 10 of the top 20 Pokemon are immune to Spikes when using their standard sets? Why am I seeing Hidden Power Fire as options for just about every single Pokemon with a usable SpA stat?
And to those saying 'oh, i can ko it with fire attacks,' that's no good if it switches into your [Pokemon without fire or STAB fighting attack] and lays down a layer of Spikes as you switch. What this ultimately boils down to is (and I know I'm repeating myself here but nobody seems to have read my first post) I think having incredibly easy and convenient access to Spikes has a very detrimental effect on the metagame. Ferrothorn doesn't keep the likes of Latios and Thundurus in check so much as it makes them even more potent by providing them with reliable and consistent entry hazard support.
Reapply this line of thought to Skarmory, and see why it fails. Skarmory 4th gen and on is VERY similar to Nattorei, as he is a strong Physical threat, has easy Spikes, powerful recovery (while Nattorei lacks this he makes up for it in bulkier stats), yet no one complains of him, and he's only weak to one type compared to Nattorei's two. How is it that Skarmory gets snuffed, but Nattorei is suddenly to powerful.
All of the centralization you pointed out was valid, and I am obliged say yet again that I completely agree with you, but this time for the fact that centralization means literally nothing. Strong, commonly used Pokemon will always be adapted to. Plain as that. The only time it becomes a problem is if people have to go out of their way to stop a threat (Notice how it's ANY Pokemon with a high enough SpA gets HP Fire slashed in for the utility, not some obscure Pokemon from NU being dragged up from the depths to stop him, but I digress.)
As to your point about easy Spikes, I say to you Stealth Rock. Whatever happened to Articuno, Moltres or Charizard? You're saying the fact that one turn can take out 50% of their health upon switch in is okay, but getting one easy layer of (avoidable) Spikes isn't?
Let's face it, easy entry hazards aren't that big of a deal. You can adapt to entry hazards by simply not using Pokemon weak to them. If you want to use Pokemon weak to them, I'd suggest you run a Pokemon with Rapid Spin. Sure, Nattorei certainly has an easy time of setting spikes up, but so does Skarmory, and Skarmory even has a phazing move to make use of them. The only difference is Nattorei is better in the meta-game than Skarmory due to typing and bulk. so he's used more often on balanced teams.
Now, I have a feeling you'll mention Froslass of UU last gen, but here's the difference: Froslass had a lightning fast taunt, easy spikes, and spin blocked for herself. Nothing you could do, short of killing her, stopped her from getting 3 layers of spikes down, and keeping them down for her team. That's pretty damn powerful for only one Pokemon, considering entire cores of some stall teams are made to do the job that she alone could do. That's why Nattorei can't compare, he simply can't pull that kind of weight.