There just was no need for a complete divorce, since neither planned on remarriage, or had a long term relationship.
As you can see from the messages posted here, most people don't seriously consider your marriage 'over' until your divorce, so to say 'there is no need to for a complete divorce' is simply not true. There are also legal implications if your spouse does something bad or stupid, which makes you a bad prospect for a relationship while still married.
And, all of this aside, I think ALL of us have heard women say they'll NEVER get back together with their ex, and a week later they do.