From discussion on user group:
http://groups.google.com/group/mongodb-user/browse_thread/thread/dfdbf0cc4a6a5891

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:

> db.testing.insert({"id":1})
> db.testing.update({"id":1}, {$set: {"data.id" : 1}})
> db.testing.find({"id":1})
{ "_id" : ..., "data" : { "id" : 1 }, "id" : 1 }
> db.testing.update({"id":1}, {$set: {"data.sub.sub.sub" : 1}})
> db.testing.find({"id":1})
> { "_id" : .., "data" : { "id" : 1, "sub" : { "sub" : { "sub" : 1 } } }, "id" : 1 }

Current behavior when field is null:

> db.testing.insert({"id":2, "data":null})
> db.testing.update({"id":2}, {$set: {"data.id" : 1}})
LEFT_SUBFIELD only supports Object: data not: 10 

Desired behavior when field is null (same as when fields don't exist):

> db.testing.insert({"id":2, "data":null})
> db.testing.update({"id":2}, {$set: {"data.id" : 1}})
> db.testing.find({"id":1})
{ "_id" : ..., "data" : { "id" : 1 }, "id" : 2 }

Maintain current behavior when field is non-null primitive:

> db.testing.insert({"id":3, "data":"non-null primitive"})
> db.testing.update({"id":3}, {$set: {"data.id" : 1}})
LEFT_SUBFIELD only supports Object: data not: 2