I also think it's a good idea, but only for those who do have an unmanageable amount of cards. I try to carry as little as possible in my wallet, now I'm at roughly 6 cards or so (2 debit/credit cards, drivers license, transit pass, library card etc). I don't think I could change this to just two cards (Coin and id) because many of the cards use NFC or barcode over here in Finland and might not work on Coin because of that.
I don't think the big issue is security, it's mostly the price and having an actually need to consolidate cards.