I think it has to do with the personality type of the individuals involved. Some folks like to keep track of everything and keep everything "fair". It is just the way they account for everything.
DH and I have wondered what happens at retirement with folks who keep everything separate. I mean, what if one person is a saver and has saved for retirement and the other person is a spender and has nothing saved. Does the saver order a steak dinner while the spender orders hot water and ketchup?
Also, I always thought that if you were married you were responsible for the debts your spouse incurred? (Maybe that varies from state to state?) So, even if everything is separate, but one spouse has mega-debt and then unfortunately passes away--isn't the surviving spouse still responsible for the debt anyway?