API doesn't really say what the purpose of the top angle is- keeping the shell round during roof erection is what I supposed.
In the current API-650, you're allowed to omit the top angle if the area furnished in that region matches that obtained with a top angle- see the wording in the top angle specification.
One other thing, I said the roof furnishes stability for the top of the shell- that's assuming a steel roof, an aluminum dome would still use a wind girder.