Can't speak for op, but for me it's a question of control. If this service ends up closing or otherwise loses me as a customer, I have to update every single contact and account before I can stop using it. That's not practical. If I bring my own domain, I can switch providers much more easily.
Some people might be ok with losing contact with the long tail after an email provider migration, but I'm not one of those people.
Some people might be ok with losing contact with the long tail after an email provider migration, but I'm not one of those people.