I think the sheer stat weightage of Arp is too much to ignore.
This is what Rawr says about the value of Arp when close to the Arp cap. These figures are from stat formulas on retail, so they might not accurately represent the situation on Warmane. The Arp value plugged into the character sheet was 1398.
As you can see, those last 2 points of arp are miles ahead of agility in value.
I understand the argument that Arp provides no benefit to your pet, nor any to Chimera Shot, Chimera-Serpent, or Wild Quiver (so about 35-40% of your damage receives no benefit whatsoever from Arp), and that its value decreases the more movement there is in a fight (less steady shots and auto shots - but more arcane shots). I just feel like a value difference as drastic as the one seen above is impossible to overcome unless the encounter involved an absurd amount of movement or AoE.
All that said, I must confess that I've never actually tried a full Agi build myself, and that it's definitely something I intend to try out at some point, if only to see how it plays out on Warmane.