Amiibos don't unlock parts of the game. They are just like additional DLC to existing complete games that have a physical toy also.
I'm going to buy Amiibos, but without a core game I think sales are going to be stunted. If the Wii/WiiU name was confusing to consumers, I think Amiibo's are going to melt their brains. I think a lot of that pitfall can be avoided if they make a big box version of Smash with included Amiibos.