For me the rarity is a odd issue.
People like Pit and Captain Falcon I like. Not enough to get day 1 originally but enough that I would have slowly built my collection and thus added them. But with the supply the way it is I have to grab everyone i even have a inkling of liking for right away or I can screw myself out of ever owning it. WFT which I thankfully got from a Gaffer here was a great catch since I thought that was done for. Marth I had planned on grabbing during christmas for me and a friend but by that time he was already scarce as hell if not basically OOS in the US. It really bothers me the way that I have to collect these now and i'm not even trying to go for a full set.
Yeah, it really does force you to buy them all now, because you don't have the option of figuring out which characters you may want to buy later. I think that is an important element that some forget, it's that these amiibos are designed to be played with in the game, it may take some time before a person actually decided if they prefer a character or not and wants to buy their amiibo. Theoretically a person could have no interest in Marth at the start, but then play around with him later in the game and finds they would love to own that amiibo. Too bad so sad.