/thf offers a once per minute crit (mostly used on WS, and I'm guessing some blue magic)
/nin you get 15% duel wield after lv50 (which more-less increases your DoT by 15%, get an extra hit on WS (doesn't equal critting it but adds damage to make up that) and shadows for survivablity (a key factor in good exp)
Remember when WARs went crazy over sneak attack raging rush? then they found out that /nin and rampage did more damage over time, oh well~
XI Wiki

