Speaking for myself, I'm excited about using B2 as a replacement for more expensive local disk in not-heavy-iops-role machines. It's not the most exotic use, but if you look at every vm provider (linode, digital ocean, even amazon) the prices of local disk are huge in comparison to the prices of cloud storage like B2 & S3.

By combining them I can have a cheap box (Linode for instance) with a local write cache, with B2 providing a terabyte of raw storage, and get a slow but effective VM with a 1TB "local disk" for under $20 month. Which is great.