Unhealthy relationships between the parents hurts the children.
A lack of good role models hurts the children.
The anger between the parents hurts the children.
These can be "truths" whether their is a divorce or not.. I know of MANY marriages that could easily be said that they have done just as much harm to the children while married.
Just because two people don't divorce or breakup doesn't mean that the relationship is healthy, doesn't mean that the parents are good role models and certainly doesn't mean there is no anger between husband and wife..
So, in some cases it may very well be BETTER for the children in the long run..
Don't be naive about reality and truth... It may or it may not be better for the children if the parents divorce... Each relationship has different dynamics...