I feel like I might be missing something, but still I don't see any reason against this approach.
If there is no value specified for LmlActor annotation, it could be resolved implicitly from related field name, e.g. @LmlActor Button button; = @LmlActor("button") Button button;.
This might be not universal since, value is array and annotation may be assigned to collection field type. But I see singular widget declaration as a general case and duplicating field name most of the time makes code cumbersome.