I've been working on my thesis proposal for awhile now, and I even took one attempt at writing it which turned out to be a bit premature. My advisor suggested that I consider making my proposal defense slides before I write the proposal. I have now finished my first iteration on the slides and found it to be a most useful exercise. The content of my slides covers what I plan to address in my proposal, although the presentation needs a unifying story to tie everything together. A presentation should have a strong motivation and story because it is important to hook the audience in the first couple of slides and to keep them engaged throughout.
My feeling is that if I start by writing my proposal, I would have a sub-par narrative that would be weak when translated to a presentation. I usually start writing with an outline, then I fill in the technical details, results, and conclusion, then write the background material: introduction, motivation, and related work. Making the presentation slides first will allow me to follow a single story throughout the proposal.
The act of thinking about how to present my work makes me consider more deeply both the organization and narrative than when I sit down to write. By expending more energy on the why and less the how, I can focus on a narrative that puts my proposed work in context and motivates my contribution. After I finish my slides, I will write my proposal using the slides as a guideline. This approach should yield a concise, readable proposal with a flow that is consistent with the presentation.
I also expect that most of my slides will be reusable for the thesis defense (and potential job talks), since the motivation and high-level ideas will be largely unchanged. Although some details and results will change, these will amount to a small delta in the presentation that will be easy to update.