Comment by progman32

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.