Should an applicant to an university mention that her spouse is currently a student in the same University, as part of her SOP?
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.