Well I'd say one reason for that is that many like to abuse the stealth the thief has with smokebomb you get the damage mitigate aswell, it's i think more usefull in many cases. Pvpwise the Thief doesn't duel that much and this skill is def. a dueling skill. WvWwise its something else but then again... just abuse the stealth its better in some cases. But in the end it's just a choice of playstyle.