I wouldn't describe it as not commonly known as much as, not commonly used. As I recall, it wasn't until 2006 that you could put post-tax funds into a 401(k) Roth. Now you can have a 401(k) and a Roth 401(k) and even have the employer match got into the Roth 401(k) (paying taxes on the match in the year of the match, of course). Once any movement includes various treatment, though, it starts to be more complex, knowing if it is basis in the destination account, was it basis in the source account, is it a taxable event, are there earnings that get treated differently, etc. Every scenario requires a matrix for evaluation and reporting. Just like those who exercised the 2010 conversion-with-carry-over option are still carrying the result of that choice.