Unhappy marriages are at least a sign that they care to try to work through their problems. The world just walks away from struggles and says "he just was verbally abusive" or "I wanted a career, he wanted a family"
My marriage has had unhappy marriage days... Should I have have divorced? As a hermit, you divorced society
If my wife didn't put up with my weaknesses and I didn't with hers, if we never struggled through our rough spots... Quitters don't prosper. I think that it's better for children to see that love means working through the struggles, and family doesn't give up on each other, be it parents/children, siblings, or whoever. it seems like some of the happiest married older couples we've met have had some very unhappy things that they had to work through.
In no way am I endorsing someone stay in the same house with a physically abusive person against their spouse or children, nor am I saying someone has to stay with someone who is living in adultery. What I am saying is that Jesus must have had some good reason to teach us about the permenance of marriage.