Given that it can be mechanically converted to JSON, I have a hard time seeing how JSON would be a lot more useful. "Very slightly more convenient" seems like the most we could reasonably say.
We might have to wait even longer for all the common law based on the court rulings to be published in the same format. Not to mention the "secret common law" based on the FISA court rulings.
Not sure what format the files are kept in, but it's been done. Several private companies have these behind a paywall:ThompsonWest and Lexis/Nexis are the most well known, but there are others. Don't forget the published caselaw of the fifty states as well as state legislation. The U.S. Code is far from the whole picture.
There are but XML schemas are well understood, standardized and have been around for a long time. Even if they did this project today, XML is the right choice due to its maturity and suitability for documents.
"Following a 1999 feasibility study on XML/SGML, the Committee on House Administration adopted XML as a data standard for the exchange of legislative documents" http://uscodebeta.house.gov/download/resources/USLM-User-Gui...
It took the US Govt 17 years to release 200,000 pages of the US code in XML.