While writing one's Statement of Purpose for applying to a grad program, should one mention that her spouse is also a grad student in the same university? What are the pro's and con's of that approach - while on one hand, it shows that the candidate is very likely to accept an offer made by the dept, it might also imply that the presence of her spouse is the main reason the applicant wants to get admitted, which might not sit well with the Admissions Committee?
This really can be a two-edged sword.
Pros: You are likely to really come and stay since your spouse is there. You can argue that you know much (positive) about the place through your spouse which has made you select the place.
Cons: You may be seen as applying because your spouse is there and not because you think the place is exactly where you want to go and be. In other words, you make a choice out of practical and not academic reasons. (largely what you already stated)
So, first of all, it is no-ones business why you apply to go to a certain place. Of course the people who read your application will evaluate your interest and so anything that strengthens the application is good anything that doesn't can be left out. Although the location of one's spouse should not be grounds for admitting (or not) a person, it is possible people read things into the fact that you did not originally anticipate. If you want to state that you already know the place is good, you may do so by demoting your spouse to a friend. I have to confess I am not at all happy about writing this which seems so negative towards mentioning your relationship but it stems from knowing that you cannot predict how people interpret things and it is therefore better to be neutral and focus on the academic rather than putting up personal reasons for a choice.
Peter Jansson's answer is the most comprehensive. I would mention the spouse to your POI during a one-on-one interview / conversation -- but otherwise keep the information private.
The rationale: The only stage at which this information could possible is important is the final rankings. At that point, we do want to know whether the student we are going to make offers to are going to come. If there are two otherwise equally ranked students on the cusp, we'd make the offer to the one we're more sure will come. The POI can make the case then that you would come based on his/her inside knowledge.
Otherwise, that information is (mostly negatively) prejudicial for the reasons that Peter Jansson mentions.
From an HR perspective, you also might want to leave out your marital status to preclude any possibility of discrimination. I am applying to grad schools now and will leave out the fact that I have a husband so that they cannot worry about my hypothetical pregnancy or marital situation affecting the timeliness of completion for the PhD program. Of course those things shouldn't be considered against you...but it's safer to leave them out, I think.