There are a few reasons why you might want to design a whole desktop publishing project in an image editor:
* Something happened to your computer and you have to use a borrowed machine for a few days.
* Your client has specifically requested that the deliverables be in a .jpg (or similar) format and you do not want to bother with the conversion process.
* You are setting up samples for your own website and you want to be sure that your samples are hard to copy so nobody steals your creations. No matter what your reason happens to be the question is can you actually do it?
Sure, you can create in an image editor, but can you create something professional that you would actually want to give a client? Well, that depends on the kind of project you are creating.