IMO, it is. Sometimes, a long shot/distance shot is not enough to kill a mob. It's left with like 2-3% HP. That's when you use fast shot, as a finisher. In this case it's worth getting only level 1 of it (if you only plan on solo playing).
However, if you choose to party play, rapid shot might prove useful for luring mobs from afar, then to be taken over by the warriors.
