Desire is like a fuel, desire can also enable you to put in much more focus, will and structure to your working. But the world is full of desire, and nothing yields from this.
The desire is the blob by which the affirmation shapes and makes it true, so the reality can be changed by it.
The affirmation is the core of your desire, the desire being the inner state of potentiality and the affirmation being the creative "kinetic" reality of it.
As for the Returning Curses, it is 18 as to permanently imprint the affirmation in to the mind and the soul. It is more on the side of a meditative working, or at least it accomplishes this function as well.
Also, the idea of affirming the energy or doing magic by just willing is a misunderstood experience, simply as one advances, one gathers creative and controlling aspect over the mind, which is to say at least a full experience of applying all of these basics into one action. But the chronological ability of this tends toward complexity, not simplicity, even if this sounds as simple.
So in short answer no, the desire cannot replace the affirmation. A working can be applied with creative capacities, yes, but the logical aspect of the working is essential, while the other part, certainty only amplifies and specifies the target. It is best as to all of the capacities of our mind and body to be put into a working. Desire, will, emotional state, affirmations and proper structure of the working, not to mention timing and time of the working.
The focus of this is what matters, and the above things are basics that need to be mastered and to become a second nature, before expanding into other forms of doing magic.