My issue with locking content behind amiibo is that in the case of DLC or pre-order bonuses, you are either pre-ordering a game that you already have interest in and would have pre-ordered regardless or are paying a nominal additional fee down the road for more content in a game you enjoy. With amiibo, specifically the Twilight Princess stuff, you are paying $13 for a toy as well as content. And to be honest, I would love to have the additional gameplay mode but I don't want another toy for my shelf. I like Zelda, but I don't plan to start a Zelda collection of figurines.
My favorite kind of amiibo content has been skins based on the characters, like MK8 and Mario Maker has done. I really only care about Sonic and Bayonetta amiibo, so it is an added bonus to see them in those games (I assume Bayonetta will at least be in Mario Maker).