ok, the change was made for many reasons, some of them internal (the current system looked fine from users perspective, but was really weird from code standpoint), but we also wanted to:
1) add more settings without cluttering CoronaMtl interface
2) make it usable in other material types (standard, shadow catcher, future Corona PBR shader, ...) without copy&pasting the entire interface
There is additional bump slot, so mixing maps is really simple
There is automatic conversion when loading older scenes, so there is no work required to convert old scenes.