Ooh yes. I feel so ripped off for those $12 for... costumes and... more costumes... and AI. Except Mario Paty. That one did lock something (meaningful) with Amiibo. 

People keep bringing the amiibo argument when everybody knows that nobody gives a shit about the content they unlock, is just an easy way to shit on Nintendo that has been doing the best DLC of the generation, period. Specially with Mario Kat 8 and Hyrule Warriors, loads of content at a cheap price.

They've been pretty great on this side. Let's be serious one minute about amiibo, the content they unlock is ridiculous. I didn't need to pay a single one and I own a load of Wii U games.

The Hyrule Warriors season pass might be the best they've released, 300h of content for 15$.

There hasn't been very much real "on disk dlc".

Splatoon's was just a remix of existing content (same map, different weapon) with a minor reward.
Smash Bros wasn't DLC content at all, but special functionality that wouldn't make sense without the amiibo.
SMM used them as a quick-unlock of content you could unlock in game anyway.
Hyrule Warriors added a single weapon, and that was added in a patch (and it's an extra weapon for a character that has far more than any other, anyway).
Captain Toad had a minor extra feature added in a patch.
Mario Kart 8 had some costumes added in a patch as a minor reward for having an amiibo.
Kirby and the Rainbow Curse added a couple of minor powerups that are completely unnecessary, just nice to have.
Yoshi's Woolly World added costumes, not dissimilar to Mario Kart 8's Mii outfits, but as special designs for Yoshi - nothing notable.

And pretty much all of the above just requires having access to an amiibo, not using it every time, so you can just borrow them from other people. Here are the only two cases where anything truly noteworthy could be referred to as "on disk dlc" unlocked with amiibos:

Code Name STEAM added three characters, as a special cameo thing.
Mario Party 10 had boards based on each of the Mario-family amiibos - and alongside that, the amiibos actually store data from the game, too, so you can take them with you to other people's houses (you can customise stuff)

Those are the only two I'm aware of, where it's content being unlocked and you need full-fledged £12 amiibos to do it, where you need to have them every time you want to use the content. So while you might complain about a couple of specific uses of amiibo in certain games, most uses of amiibo are nothing more than little extras you get for having access to an amiibo, not having to keep one indefinitely. They're certainly not "on disk dlc that you can only unlock by buying a £12 piece of plastic" other than maybe those two cases.

I've left off a few specific cases along the way - but I did so because they're not even worth talking about - quick access to tokens, for instance, or unlockable demos in a free download.

I think they did pretty well with DLC, I have only 2 complains:
1-No Battle Frontier DLC in Pokémon ORAS is just meaningless;
2-Some FREE DLC characters in Smash would have been a nice addition (imho Lucas+someone that'll eventually come with the smash ballot results)

They did mostly fine. Some things were a bit excessive (Triforce Heroes is going to be one of those, I'm sure), but overall they've pretty much hit all three levels of DLC, from bad to good.

If no one buys amiibos for the extra content then there's literally no reason to lock content behind them since it wouln't affect amiibo sales.

But then you cut the relation toy-videogame, and both need each other, the videogames help to sell more amiibo and the amiibo help to sell more videogames, is a positive feedback, the content they unlock is almost symbolic, it give adults a reason to buy toys they want without feeling weird with themself because they have functionalities with the games even when they add very little to them.

Mario kart was done very well.

Splatoon was free, but you could argue that the dlc should have been as part of the original package (and not unlocked at Nintendo's discretion) to begin with given the $60 price tag.

Pikmin 3 had solid dlc, nothing wowing, but worthwhile and decently priced.

Smash dlc was horribly done. You pay almost 1/3 of the game price ($20/$60) to expand your character roster by about 8% (4/50) and get 2 extra stages. This, along with the multitude of overpriced costumes they want you to buy is insane. And I don't care if other fighting games do this. Just because others do something wrong doesn't mean you should.

Fire emblem awakening dlc is also horribly priced.

Finally, amiibo is a dumb way to lock content given that some are ridiculous to find and cost at the very least $13.

Nintendo is not the worst when it comes to dlc. They have good moments, and very bad moments. Overall, I would rate them as 6/10.

So then people do buy amiibo to unlock content?

You can't assume that all people buy amiibo as a collectible and have the in game content as a "bonus." For all we know, the reverse could be true. People may just buy a smash amiibo to test out the AI learning capabilities of the amiibo characters. In this case, it is paid dlc, which is kinda dumb