Depends of Mentality of player. Thief can Be or No A DD.
Thief have all arsenal, gear, Job abilities and strong WS. For be a great Deal damager.
Even having all this and post abbysea. Depends of the player mentality the way thief will be used.
Examples: People is lving monk for tank only, they dont have plans of parse his monk in einjerhar or dynamis. People only lv up whm for use that only in abbysea, with infinite mp and for low man with friends NMS, they dont have plans for take a main heal in dynamis or in einjerhar. The same problem is with thief, people just lv up that for tank weak NMS and TH.
If is a player with Thief career. The thief willl be a great deal damager.
In abbysea a skilled Thief, can keep up in damage with any competent DD Warrior included. (Talking about people know what is doing, swaping gear, having haste capped, having good atmas. Not talking about full pinky, or full perle gear DDS)
Outside of abbysea, in einjerhar, dynamis, limbus. a Skilled thief can Keep up but not outparse a equal skilled player using Two handed job.
Sadly but outside of abbysea a 4 hit build sam, a 5 hit build drk, war and drg are superior than DW jobs.