Benefits
(a) They care / theoretically have your interests at heart / aren't going to embezzle / aren't to leave
(b) "Tax" Benefits ie. 401k, health insurance
(c) If you married someone on your level ie. you are an intelligent person, you married an intelligent person then you may have someone who is capable of seeing the big pictures, handling more complicated issues like billing / remittance / potentially more teachable. Teaching an employee to resolve complicated issues can be difficult, expensive etc. I think we have 1 employee who went to college ie. our OM.
Cons
(a) Its entirely possible that they are bad at it and have no business being involved in your business
(b) Its possible they have something they are good at that they should be off doing that could pay more, be more satisfying, be more worthwhile
(c) The overall experience of people on this forum is that nothing is worse than dealing with an office manager who is also a podiatrist's wife. ie. they run associates off, often treat staff poorly.
I think there's some truth to this regardless of the business. Family members who "work" at an office often speak with the full authority of the owner without actually working, contributing, etc. My wife worked for a business where the owner's spouse would simply show up, yell at people, complain / point out tiny problems, and then leave.
One of my complaints about someone I used to work with was that they never had a complaint that would actually make the office a dollar.