You have a good point that user_name validation is the same whether or not you use a DB or not, however, if you have been leveraging an RDB for doing validation

You have a good point that user_name validation is the same whether or not you use a DB or not, however, if you have been leveraging an RDB for doing validation, queries and collection management, then making the transition to doing all of your work in objects (i.e., using an ODB), means that you need to start thinking in terms of objects everywhere. This turns out to be a fairly common question from folks transitioning from RDBs