I think that is a question best addressed by an adcom who reads the applications for a living.
@Goro @gyngyn @LizzyM would you please be willing to weigh in on his/her question?
My own two cents (if I happened to be a student reader) would be that if your partner was a legal spouse it would hold more weight than boyfriend/girlfriend, even if it was a long-term relationship. It would also depend on their profession and mobility of said profession. I believe if they are already a student/faculty at the school you are applying that would be worth mentioning. At the risk of repeating myself, I would turn devil's advocate again--If the two of you were to break up, would you still want to go to that school above others? If so, why? Is it the only school in that city/region?
Ultimately I am a horrible cynic who likes moving from place to place; I might be more difficult to convince than others that a significant other is a good reason to be in a place. However I know many school's do not wish to train someone up and have them move across the country to somewhere "more desirable." If you truly wish to build a life in your school's city/region, it can't hurt to mention that.