Holy is not only viable for healing, but also for dealing damage. These paladins (so called Exordins (previously Shockadins)) cast spells to harm their enemies. The paladin attacks enemies with spells such as [Holy Shock] and [Exorcism], for which [Denounce] is crucial: Holy dps can't be found under level 31.
Paladins are one of the four classes in the game that can be adapted to absorb damage like a sponge and keep others from harm. They are particularly famous for the ability to tank multiple (10+) mobs at the same time. Besides, Paladins are also well known for their ability to survive with their unique combination of invulnerability, plate armor and healing spells, which makes them very good for AoE farming.
Retribution paladins are excellent DPS offenders in PvE and PvP since the major retribution buffing in Patch 2.3.0 and Patch 3.0.2. The instrinsic high survivability of paladins coupled with the potential of high burst damage provided by the Retribution tree make them capable of disposing almost any classes one on one and valuable both in dealing and absorbing damage in group PvP. In PvE, besides the high damage output, they also provide various useful utilities to their party members.