Stop your code from shattering.
Our code is put under immense pressure
Fat JavaScript apps enforce a separation of concerns between Ruby domain logic and the presentation layer
When we refactor a model we have to make changes in the views by default.
When we build our models, we have the presentation in mind. Details on how the views are structured constantly leak backwards; often even determining which views we have. We keep view methods in models.
JavaScript is getting better. Angular, Ember, and modern tooling like Lineman. Default Rails encourages entanglement of models and views.