Just to expand on this: I wrote the first draft of my Statement of Intent focusing on the prestige of NPSIA and my future career goals (liberal internationalism, "golden age of Canadian foreign policy," you get the picture). I showed it to a few friends - beyond a few grammatical changes, no major revisions needed, in their eyes. I showed it to one of my academic references - a change or two, but no major changes. I felt great about what I wrote and thought I was a shoo-in.
Well, I showed my statement to my other reference, who sent me back comments with no fewer than four paragraphs needing to be deleted, and the remainder being scratched heavily. We worked on my statement together afterwards, but his main advice was: NPSIA is a research school, so focus on research, and the skillset you have that would lead to strong research. Think of your statement as if you were applying to a research position: what experience do you have that will make you a strong candidate, and what drives you to want to attend the school?
Here is a brief outline of my Statement:
P.1.: introduction/personal background
P.2.: research experience #1 (fourth-year research paper I wrote)
P.3.: research experience #2 (third-year research paper)
P.4.: professional experience (identify the skills you have gained here that would serve you well)
P.5.: objectives (what courses do you want to take? Be specific, down to the course code. What do you hope to accomplish at school? I.e. internships)
P.6.: conclusion (here, I expanded on future career goals briefly)
I don't think a weak statement would keep an otherwise strong applicant from being accepted, but I do believe a statement where you present yourself as an asset to the school can significantly boost your application. My GPA was not phenomenal, nor do I have international experience. I truly believe aligning my statement with the school's interests, and making my statement about the work itself, made my case much stronger.
Hopefully this helps, OP!