As for your question regarding practical purposes I think that a blockchain run by multiple banks or states can provide the same benefit for people as a fully decentralized system. I get that this is where censorship and control comes in but do you really want to live in a world where value can be moved around arbitrarily without regulation?
Also, while the blockchain itself is run decentralized the development is still somewhat centralized and most folks do not have the understanding or interest to investigate changes being made to the source that powers such blockchains.