And what about noSQL databases?
NoSQL is entirely a buzzword and has no meaning whatsoever. Asking if they will support NoSQL is like asking if they will support about 30 different databases with completely different structures and APIs.
Everyone says that they are faster and more scalable than SQL.
SQL isnt the problem here, its ACID. If you throw ACID out the window, there are a ton of things you can do to increase performance, which is the reason some of the NoSQL solutions can benchmark so well. If you are doing something with financial software, not using ACID is about as stupid as you can get (for the actual financial stuff, maybe for the blockchain where you have a bit more leeway and are only updating once every ~10 minutes anyway...)
Moreover MongoDB have interface in JSON, so it fits with default bitcoind interface.
I can't speak for xelister, but I dont think supporting JSON matters here as its a whole new lib anyway so whether or not the original bitcoin client supports JSON or not is fairly irrelevant.