While it's largely blather in the vein of a frightened goose honking at something it doesn't understand, I concede that there's a valid challenge to my logic in there. Was Mongoose the wrong way to go for an enchant on my Felsteel Reaper?
First, consider that I'm using a 2H axe for dps. My rationale for that is as follows:
The majority of my damage comes from Raptor Strikes, which can only be done every six seconds, so I want to get as much as possible out of each Strike when the cooldown is up. With 2/2 points in the Savage Strikes talent, I have an extra 20% chance to crit on a Raptor Strike, in addition to the +170 damage it deals. The slower and heavier my weapon, the bigger those crits will be. That's very important when you consider that we're talking about my strongest attack.
So if I'm sticking with a 2H weapon for now, there's really only three solid enchants to choose from: Savagery (+70 AP), Major Agility (+35 agi), and Mongoose (+120 agi and attack speed...sometimes).
Disregarding Mongoose for the moment, I'm left to choose between 70 AP and 35 agi. rmlinden on Thottbot says it plainly: "Simply put Agility scales AP. Static AP doesnt." Consider what 35 agility (40.25, thanks to Lightning Reflexes) means to me as a hunter. More AP (40.25), more crit (0.96%), more armor (82), more dodge (1.64%). Dodges unlock Mongoose Bites, which are also subject to the 20% increased chance to crit from Savage Strikes. In turn, crits unlock Kill Command. The added chance to crit, Bite, and Kill represents a lot more damage than the additional flat 29.75 AP Savagery would provide over Major Agility. Major Agility wins.
So now it's between Major Agility and Mongoose.
Mongoose seems to be normalized at 1 PPM. Roughly once a minute I'm going to get +120 agi for 15 seconds. Averaged out over that one minute, that's four 15-second blocks of +30 agility. Looking at it that way, yes, that's inferior to +35 agi. I'm coming up 5 agi short.
Now comes the tricky part. The slower the weapon, the more often the Mongoose enchant will proc from instant attacks like Wing Clip and Counterattack.
My base chance to proc Mongoose is ((1 PPM * 3.4 weapon speed) / 60) = 5.67% chance per hit. That same 5.67% chance to proc also applies to my instant attacks, like Wing Clip. I've talked before about using Wing Clip to proc effects, and this is why. At least every two seconds, I'm hitting a special attack. If I haven't gotten off a Counterattack or a Mongoose Bite recently, I'll hit a Wing Clip. So at a minimum I'm doing 30 "bonus rolls" per minute for this proc, and I should be seeing a proc every 18 attacks or so. When you combine the base 1 PPM from my white damage with the additional chance from my special attacks, I'm actually averaging two (or more) Mongoose procs per minute, not one. That takes my minute's worth of four averaged-out 15-second blocks from 30 agility to 60, greatly exceeding Major Agility's 35.
Tl;dr -- Mongoose wins.