A clear business model and alignment of interests, as a customer. Also no lock in since the customer owns their domain.
How is SDF funded? How can anyone be certain it will continue to be funded, if it offers a service gratis that costs the provider money to operate? Is that funding source aligned with my beliefs and interests? These are questions you don't have when you just pay for things.
Another example: I use FastMail with a custom domain. Yes, I could just use Gmail, but I don't, for the reasons listed above.