Advantage of traktor with the CDJs is the fact you dont have to go through the rekordbox analysis and exporting , plus with traktor you have added effects that you may want to use. Cdj 2000s have a bigger screen so navigating through tracks is easier if you have loads on your usb. The 900s have a crappier screen that I hate navigating through .
If I'm not mistaken, you have to go through traktor analysis to use traktor much like you have to do with rekordbox or serato. Exporting, well, that's if you want to leave your laptop at home and just turn up with a USB. If you like using your laptop, just put in an Ethernet cable and drag/drop tunes to your decks. When I'm playing at home and want access to my entire library, that's how I play. When playing out, I export what I want to a USB (actually 2 or 3 just in case), but it's straight forward and then don't have to muck about with setting up a laptop in the DJ booth.
Remember those days when you used to have to agonize all day on what to bring then lug 50lbs or records to a club. Then once you are on and you wanted to switch tracks, you'd have to bend down and flick through a crate to find what you want, blah blah blah. Now people complain about exporting files to a bloody flash drive.
The fact of the matter is if you are using CDJ-900s or 2000s, then need for traktor/serato just isn't there. They loop beautifully and no one wants to hear a DJ layer on a bunch of god awful built-in effects. The ones that are available on every modern DJ mixer are bad enough.