Using blockchain ontop of IPFS is to provide incentives. I don't know what is "questionable" about Filecoin, it seems fine to me. I used IPFS before blockchain but mostly just to play around with, never for any serious projects. Filecoin is simply a way to get more people to use IPFS. NFT.storage is the same, they see a market they would be a perfect fit for and try to get more and more people using IPFS.