Work Spouse: detrimental or healthy?

Not sure if everyone knows what a work spouse is, here’s a Wiki article on it http://en.wikipedia.org/wiki/Work_spouse

A work spouse is a co-worker (usually of the opposite sex)[1] with whom one shares a special relationship, having bonds similar to those of a marriage; such as, special confidences, loyalties, shared jokes and experiences, and an unusual degree of honesty or openness

Do you have a work spouse? How does your actual spouse feel about this, or do they not know?

My mother and I were discussing this, and she was telling me she has one..but they are never inappropriate with each other and do not complain about their spouses since they are both madly in love with their spouses. She says they just joke a lot, as they have cubicles right next to each other, and he can even guess whats for dinner when she mentions that her husband is not home (my mother cannot cook, she only knows about a handful of dishes. lol)

Why do these sort of relationships form? Is it because a spouse just cannot understand the work issues because it’s not their field/job? Or because their spouse is uninterested in their partners work?
Can it be harmful to a marriage?
Thoughts!