I think Rethink is general enough that they don't need use-cases listed on the site. It's not an in-memory database geared towards real-time analytics, it's not a clustered map-reduce system for number crunching, it's a document-based database with real durability and a great query language. Use your imagination.

You're right though about the fear of putting your company's data into a new system. However, I think they've taken the right approach by warning people of the risks while at the same time working as hard as they can to mitigate any of the risks.

Getting people to trust you is a slow but steady process, and I think they're playing it right.