In an app, I can import a picture either through attaching an image or pasting a URL in a field on the UI. That URL field does not exist in the DB, but in order to benefit from ActionView's helpers, I had to declare the field on my model, even though the value in that field never goes to the model. The controller takes the field's value and does the right thing with it.

This is a very simple example of a common task that the separation between UI and backend fails.