Righteous Fury is absolutely necessary for Paladins who are Main Tanking to ensure adequate threat generation. The 6% damage reduction from the Improved Righteous Fury also makes a significant difference.
This increases the threat generated by all holy spells, including healing spells.
Depending on the fight and spec, healing with Righteous Fury active can be either good or bad. Numerous higher tiered raids have large amounts of AOE damage, where the 6% damage reduction from Improved Righteous Fury can save a healer's life, and healing aggro is seldom an issue. However, many fights include aggro wipes or reductions, and 6% damage reduction is not worth pulling off the tank. Keep an eye on the threat meters and know the fights before activating Righteous Fury as a healer.
Activating this spell with a Retribution build will deactivate the threat-reducing properties of Fanaticism while the buff is active, so dont waste your talent points on both Improved Righteous Fury and Retribution tree threat.reducting talents.
Patch 3.2.0 (2009-08-04): No longer has a duration, remaining until cancelled or death. Also cancelled when a Paladin activates a different talent specialization.
Patch 3.2.2 (2009-09-22): The bonus threat from Holy spells caused by this talent has been reduced from 90% to 80%.
Edit
Patch 3.2.0 (2009-08-04): No longer has a duration, remaining until cancelled or death. Also cancelled when a Paladin activates a different talent specialization. 
