By far, this weapon will be BiS for tanking DKs. Here's why.
- No other weapon has a top end like that, so all your physical attacks will be increased by an amount that no other weapon in the game so far has. That alone warrants it BiS for threat.
- The 'proc' component if it is led to believe that it is what it is right now, will add more AoE threat and again, no other weapons in the two handed directory has that proc. Again, BiS.
- The 'proc' also boosts up your damage by giving you the single stat that gives you the most damage to your skill: Strength. No other weapon will give you 360 Strength at the end of the proc. That's more than the Darkmoon Card: Greatness was giving you.
- You can effectively increase the weapon's health pool by nearly 100 points by putting three sockets in the weapon purely Stamina. As we well know, most (Read: not all) weapon sockets are not the most amazing therefore you maximize your damage potential by adding whatever you want. In the case for this weapon being a tank, you can effectively maximise your Effective Health, making it (and I'm taking a shot at the dark because I haven't see the Season whatever they're at pvp weaons) one of the prime BiS weapons for effective health.
- The sheer amount of strength you can put in gives you a sizable avoidance potential. Placing Strength in the three slots instead will net you nearly 300 Strength, which is well over 12% extra Parry. That's WITHOUT calculating the proc which will add another 4% more Parry. A weapon that can potentially net you 16% parry without diminishing returns affecting it is not one to scoff at and makes it quite the avoidance weapon to boot.
To end this: Yes the stats on the weapon that they increase are not in any means a tanking stat. There's no dodge, no parry, no defense, not even expertise. But to say that the weapon itself would not be BiS for DK tanks is having your head stuck in the sand without really wanting to see the truth.
Looking at the ICC weapons and what was found so far (on a rather bad datamining site, mind you), there is one weapon that has a 'tanking' proc to it which steals life that could be considered the 'prime' weapon. But it has two slots, and much less than the Legendary does.
I'm still going to maintain that the axe is BiS for DK tanks.
But like some people have mentioned, it will come down to who can use it to the maximum potential in a situation where it may arise to min/max. Some guilds will figure out their own way to give the Legendary. My point in this whole thread is to try to make people realize that DKs are unique in this position because they have two handed weapons to use and as such, EVERYTHING is fair in the game to go after. The reason? Because Blizzard wants you to use the damaging Two Handed weapons.
Simple as that.