This is specially interesting and it would be important to make it right.

One of the features we lose in Wayland is that we know where to place each component. because of this number being 0 the primary screen, where the important components go. In Wayland there's not a concept of a primary screen and using multiscreen there is basically gambling. One thing we discussed with @mart at some point is to use the screen name to identify the configuration.

That's another use-case for the API.
That said, I think that it would be just better to slowly deprecate the integer id.