Usability of 1:1 relationships stretches far beyond UI candy. They can become a very useful tool that secures parts of the same logical record. For example, financial services company might use account entity to hold information about their customers but due to Chinese walls within the company, investment manager should not be able to access tax profile of the customer while taxation specialist should not have visibility of the investment details.

Create two new entities, Tax Profile and Investment Profile, create 1:1 relationship between Account and each of these entities then configure and apply appropriate security roles that limit access to one or another profile entities for specific users. That way, investment manager will not have access to tax information (or will have read-only access only, for example) while taxation agent will see bupkis instead of Cayman Islands account details.