I suppose row-level locking is a necessity when you want to be able to scale vertically. By comparison, I understand that MongoDB doesn't even try to support vertical scaling so it makes sense to not bother with complex locking systems.

Personally, I'd prefer to have to option of scaling without being forced to increase the number of moving parts in my single most important system (the database)...but the MongoDB docs make the fair point that there's a ceiling to vertical scaling. It's probably higher than most people think though.

As an aside, I discovered recently that Informix supports byte-level locks for 'smart large objects' (namely CLOBs and BLOBs). Makes me wonder whether field level locking will appear some day.