Principles of User Interface Design

I think the principle about a "Single Primary Action" per screen heavily depends on the type of app you're building, and which use case you're optimizing for.

Sometimes, especially for power users of an app, it's useful to have multiple actions at their fingertips, even at the cost of introducing a learning curve.

I think the one primary action per page thing works well for most things, but as soon as you're building anything like creativity tools where people want multiple editing options, or a control panel that you expect will be used by a professional several times a day, I think that principle becomes less useful.