From discussion on user group:
When performing updates involving subdocuments, it would greatly simplify the edge cases which need to be handled client-side if the server would automatically create the necessary subdocument(s) indicated by the "dot" notation in the update when fields in chain are null (instead of the LEFT_SUBFIELD error that is currently returned). This would align the behavior more towards how it currently behaves when fields in the chain do not exist.
However, I would expect the LEFT_SUBFIELD error if a field is currently a non-null primitive since you wouldn't want to automatically overwrite that primitive data (which is current behavior).
One common use case where this would have a very positive impact is in data migrations. In migrations you often may need to replace a null field with a subdoc as schemas evolve but may not know (or want to worry about) whether that field is currently null (preventing sometimes expensive/complex document introspection prior to updating).
Current behavior when fields don't exist:
Current behavior when field is null:
Desired behavior when field is null (same as when fields don't exist):
Maintain current behavior when field is non-null primitive: