Allowing the application to specify its preferred tradeoff of availability and consistency lets apps do what is best for them (#)
Key-Value is a better fit for some application needs than relational, and allows for certain desirable operational characteristics (#)
talk-deconstructing-the-database
Splitting process and perception allows you focus on each individually. For process, you can ensure a transactional processing of writes. Indexing of the immutable values can happen after-the-fact. (#)
Data immutability allows you to scale readers and have aggressive caching. (#)
You can distribute the eventing and reacting to the "peers" (#)