Sounds like a recipe for disaster to me.
If it doesn't last, then it can cause irreversible harm to the children. They can be seen as reminders of a painful past by the parents, along with the fact that children may think they caused it to end and it's their fault everything is messed up now.
If one of the parent finds another partner, what happens if they break up then? Children constantly going through changes with their families, the people who raise them, isn't the best for them.
Plus, what if it develops into something more with one of the parents? What if they want more but the other doesn't? That's more harm caused to the parents, which can then impact the children.
Don't have kids just because you have baby fever.
Sure you don't have to pay for a wedding or a divorce, so costs are cut down there. That's really the only good I can see out of this.