Well, as far as I can tell, the things Amiibos add to games aren't anything really integral to the game itself. You're not losing full characters and shit by not buying up every Amiibo ever made. I don't know what every amiibo does for each game, but for instance the Smash Bros series adds in a computer controlled character that levels and whatnot. It fights for or against you. Not entirely sure what the benefit of this is really - but it doesn't prevent you from selecting the character yourself to play as. In Splatoon it unlocks additional challenges.

I tend to like them more as figurines anyway.