But it can be much more useful to have Tarth's double strike and additional attack and initiative for a ranged unit instead of the one time rush ability. Just as the Wraith bloodline can be much more useful for an Assassin build or the Ironeer bloodline can be much more useful for a Defender build, etc.
All I am saying is that the rush ability may be very nice and strong, but it is a situational power that will not always translate into the best option in a certain situation, you can perfectly build other tactics around the abilities of the other bloodlines.