Just my 2c, thief is very viable in pve. After trying all the other classes available in the game, I mained thief for obvious reasons: they have low skill ceiling (compare to most classes) and yet high burst damage (kill very quickly) and still high in survivability. Plus they are not boring and cool looking. Yes, I just love my thief. They are kinda selfish, but if my group want to clear dungeons quickly they would appreciate me.