The Re-Frame docs strongly encourage the use of Spec, but I guess having all your state in one place is going to help.
Yes, very good point. Having data specs for the app-state would be a very helpful development tool, as would checking you are sending the right data as arguments to functions. Using specs to generate a wide range of test data would also making testing a lot easier (especially compared to testing via the UI). Hopefully I'll find more time to look at re-frame over the summer.