I don't think you can compare what the average studio guy does and what Justin does. Justin has made a name for himself as an experimental popular music bassist with a heavy heavy groove. As far as I know (correct me if I'm wrong, Justin), he's not getting hired to read the bass chart for the latest Applebee's commercial. He's getting hired to be Justin Meldal-Johnsen for the most part.
Again, if I'm out of line here, please set me straight, but I don't think of him as the traditional studio guy like Joe Osborn or Carol Kaye.
And I wouldn't use what Justin says about not having to do much sight reading as an excuse not to practice your sight reading. He's JMJ, we're not