"Why bring in a bunch of lawyers? Why create rancor when there’s nowhere to go but down?"
Plus, there are all sorts of financial reasons to preserve the marriage — health insurance, pensions, joint tax filing status. If you both understand what you are doing, why isn't this a great solution?
1. One spouse may be more attached to the old relationship than the other and may see this failure to divorce as hope for getting back together. You might need the divorce to express that the relationship is truly over.
2. It makes it hard to establish relationships with new partners, who might feel uneasy or immoral consorting with you. You might need a divorce for the peace of mind and comfort of those outsiders to the marriage.
3. Maybe it's not ethical to collect the financial benefits of marriage if you aren't committed to each other in the traditional way, living together and having an exclusive emotional and sexual relationship. Is it as wrong as 2 individuals marrying solely to create an economic benefit and without any sex or love at all? But how wrong is that?