Further clarification:

DTR's proc chance recasts a spell you have just cast.
Atiesh, Thunderfury, Sulfuras and Shadowmourne all gave STATIC PROCS of stats or damage
DTR, because it duplicates a spell, scales directly with the rest of your gear.

That means that while you upgrade your gear in the Deathwing raid YOU ARE ALSO increasing the damage of your DTR procs.

The fact that this occurs means that some theorycrafters said it would even be viable well into 5.0, barring an absolutely massive amount of stat inflation.

Item levels do not increase significantly enough between tiers to replace a weapon that scales alongside all the other pieces of your gear with one that as a slightly higher number of static stats, because the one that scales with your gear is effectively using the stats on the weapon AND a certain amount of the stats on everything else you are wearing

This means that if the staff had not been nerfed it could potentially have been MORE useful this tier than it even was in Firelands
That is why it was nerfed