Deis = heroku-like interface on top of your own private cloud which must be running CoreOS and certain other things like Ceph distributed filesystem, but can be set to run on a variety of third party commercial cloud providers' infrastructure.
So basically, if you only ever want to use these features, and you are happy with a heroku-like interface to your infrastructure, Deis is useful.
OTOH, if you might want to run a non-Linux node, a specific filesystem or node-local configuration for performance, security or other purposes, or have infrastructure that is not feasibly manageable with a manual process, Deis is not currently useful.
That said, neither are most of the latest-gen infrastructure solutions.