In my first getting started post on HTML 5 canvas I made a simple example on how to quickly throw together a canvas project in a way in which I usually do so when first starting out with a canvas project. However the way I wne t about things there is not always what I would do if things start to get a little complex. As a project grows in size it becomes more important to separate code that has to do with creating and updating a module from code that renders that module to the canvas.
So in other words if a project is something stupid simple things like separation of concerns does not strike me as something that is that important. However if I do start to put together something that is a little advanced, it does become more important to separate what is often called state, or a model from what is often called a view. Doing so is a good way of avoiding writing the dreaded spaghetti code as a project advances.