For those short on AP, leaving AR at rank 2 for a bit could be time-saving. (60 AP for one rank? wtf!!?!? orz!!!)
That being said, unless the player uses magnum shot alot, it shouldn't be a priority to rank, as it doesn't give dex bonuses.
With support shot, I believe melee damage is +70%, not +200%. Besides the fact, I don't think it's very efficient to use it solo. You'd have to do something like Support Shot + Switch weapon + Windmill + Defense + 1 + Switch Weapon + Support Shot or something.
Compare Critical Hit and Support Shot. The additional damage multipliers listed are 150% and 200% respectively. Support Shot can triple melee damage with the mastery title. Players donning the Bohemian Set can further increase the effects of Support Shot.
This will be a party/boss killing skill.
I can see something like, support shot > pet swipe > smash being viable. That's an easy 1800% damage multiplier.