Bane increment can be useful in certain situations, sure. Having bane increment is a good thing for the occasional HHK or Rust exp group. However, I don't think adding a focus to a high-end raid tiered item just for the sake of making the occasional exp group more awesome is a good idea. You have to keep in mind that Thaz armor was made with the intent of it being used by high-tier raiding guilds in high-tier raiding situations. In those situations, that is, general raiding that does not involve undead(pretty much every bit of raid content past some parts of Factory), ferocity(looking at the boots) is going to improve paladin DPS across the board much more significantly than bane increment. Seeing as paladins are the "DPS tanks" of the game, I think it makes perfect sense for the top-tiered raid gear to build upon that concept in general, rather than nitpicking itself down to a very few zone instances for AAing. The same can be argued for FT and its necessity in keeping paladin mana up for group HoTs.
In short, I think it's a bad idea to take endgame raid gear and change it up so it will be utility for awesome HHK groups. It needs to stay utility for awesome endgame raid DPS.