When we both had our own jobs, we felt like we hardly saw each other and like we had nothing in common. Now that we have kids, it's much easier to have the kids around while we're working because we can both do more or less as the kids need us.
Obviously if we got divorced it would be a disaster, but that doesn't negate that these are currently very real advantages.
Advantages: Complete trust. We have a lot of experience with difficult conversations. We have complementary skills and personality traits. We have both bet our future on this company and thus are equally dedicated. We're also aligned on motivation to succeed.
If you are willing to entangle your life with someone financially, romantically, and domestically, you may as well start a business with them :)