The rogue daggers have a few things to consider.
1. The proc wasn't very good a lot of the time in pvp. Sure, if you got it to go off at a good time, it could be devastating. But if you get ccd, or your target blinks/sprints/otherwise gets away, or simply dies as it procs, the six seconds of free finishers can pass by without getting a use. This is compounded by the fact you have no idea at how many stacks it will activate. The stacking agility buff that precedes it is probably better in pvp that the final proc.
2. The ilvl wasn't any higher than other heroic Deathwing weapons. However, I think the daggers got a bad rep because you didn't HAVE to raid herocis to get it. My guild never touched heroic spine or heroic madness, and yet we managed to get FOUR daggers out before we stopped raiding for the expansion. I had the first one, completed before we even started heroics, I think the second one might have too. So I think the complaint wasn't that it was the best weapon in the game, but simply that it was far more accessible than it should have been.
At least in 5.4 we won't have this problem, since not only is the questline to even get to 5.4's wrathion quest much longer than any of the others, spanning multiple tiers, but said legendaries will be brought down to conquest gear ivl anyway. Maybe the procs will still be strong though, we'll see. But they've nerfed pve trinkets and procs in pvp, why not legendary procs too?