Why do you people keep saying that? Not everyone has enough money to spend on themes that literally don't do a single thing! I want a couple of themes, but instead of paying the 12€ to get those few I want (just 5 themes), I can just go and buy a game or two (or even more) from the eshop!
I mean, that's just how I feel on the matter, my opinion on it, if you will. Everyone's going to feel differently on this. You're right that not everyone has enough money to spend on themes that don't do anything. I personally don't buy themes because the few that I like don't really seem like the little bit of money they're asking for. The ones I want are only about $1, and I could easily buy them with the money on my eShop account, but at the same time, I don't want them enough to spend $1 on them either. The only theme I've ever bought is the Slowpoke one, and that's because I really like that one and felt like it was worth my money (and I had extra money on my eShop account from a card a former friend gave me as a present a while ago). I did a conversion of 12€ to USD, and that's about $12.89 USD, which is pretty pricey for the 5 themes you want, yes.
No, they don't do anything. No, they're probably not worth what Nintendo's asking for most of them. No, not everyone has the money to buy these things or is lucky enough to receive an eShop card as a gift like I did. (If I hadn't received that eShop card, I wouldn't have the one theme I bought, nor would I have cared enough to go out of my way to buy one myself for the theme.) But no, Play Coins shouldn't really be an option for buying themes at all. Maybe some of them, like I said in my earlier post, but definitely not all of them. I think Play Coins should get more usage, since they can only really be used in some games, but I don't think this is the right use for it, nor am I really sure what the right use for them would be.
I don't think there's really a reasonable conversion for real money to Play Coins either since as far as I'm aware (aka based on the little research I've done while typing this tl;dr post), one can only have 300 Play Coins in their bank at any given time anyways. Nintendo would have to come up with a reasonable conversion of real money to Play Coins in order for them to continue gaining profit off of the people who want to pay to buy themes or get themes the "free" way by earning Play Coins to purchase them instead. (Free is in quotes because they wouldn't really be free since one would have to put time and effort into actually getting the Play Coins.)