This was the paladins ""tanking"" set pre-tbc. Altough some pieces could be used with different specs. ""Working as intended""

As of 4.0.1, This is now a half dps, half tanking set.

AQ20 sets seemed to focus on gearing unusual speccs: Protection Paladins (or Retribution for that fact) were barely heard of back then.
Protection Paladins, back in Vanilla, didn't have the mana and spell damage improvements that come with the specc nowadays, so they weren't as optimal as Warriors or Druids at keeping threat (even with Righteous Fury). The set bonus made sense back then, because most often than not, the proc would provide more mana than which was spent.
The sword has an unique model (only shared with the mage sword), with a great look, so it's definitely an item to grab for transmog.