Why does Righteous Fury have a duration? As a protection tank, this irritates me greatly, because its one additional thing I need to pay attention to. I mean, warriors shift into defensive stance, druids go bear, and deathknights shift into frost stance. None of them have to worry about whether their threat is going to suddenly fall off because they forgot to refresh a spell.
I realise that the 'stances' that each other tanking class offers advantages and disadvantages, and that there aren't really other 'stances' that paladins have (I do NOT want our increased threat generation to be tied into our auras), however it would be nice if once we cast Righteous Fury, it would stay up until I manually clicked it off. Would that be so game breaking? Its not like its something we have to cast every pull, and its not like it's a major mana sink or anything - it just seems to be an old dinosaur from original game design that should be allowed to go extinct. Having a half hour means that if I'm not paying attention, I could wipe a group simply because I forgot to refresh the spell before a pull.
There are really two issues at play here. First, why do buffs in general have a duration? Second, why is Righteous Fury a buff, rather than a stance, form or aspect?
Let's take the first question: why do buffs have a duration? Buffs have a duration because it tests skill in a very small way. All other things being equal, a paladin who remembers to refresh Righteous Fury is simply a better paladin than one who forgets and lets it wear off. Part of playing a class well is maintaining your buffs.
You can see this very clearly with paladins and Blessings. We've all run with paladins who let their Blessings expire and have to be prodded to re-Bless. Then at the other extreme you have the paladins who keep everyone buffed all the time, who hits battle-rezed players, pets, and even warlock imps. It's a small thing, but---all other things being equal--the second type of paladin is a better player.
Not all challenges need to be amazingly hard. Small, easy challenges such as keeping buffs up are still important. Now, these small challenges need to be kept in check. Overused, they become extremely tedious and detract from more important and fun challenges, as anyone who remembers the days of 5-minute Blessings and 40-man raids will attest to.
On to the second question: Why is Righteous Fury a buff, and not a stance?
In general, WoW design likes to use buffs for mechanics which are purely additive, and use stances for when you make a choice between two effects or for an effect with both a positive and negative aspect. Blessing of Kings is purely additive, adding 10% more stats. Similarly, Righteous Fury is purely additive, adding extra threat. While warrior stances offer a choice between extra damage dealt and less damage taken.
It's more because threat is sometimes a positive stat from the perspective of the player, and sometimes a negative stat from that perspective, that Righteous Fury feels a bit different from all the other additive buffs. Essentially, there's no built-in negative to Righteous Fury. The negative doesn't come from the buff, it comes from the nature of the stat the buff provides. Compare this to Moonkin Form, where the negative--can't cast healing spells--is built into the form, and is not a side-effect of extra critical strikes.
For example, if you look at PvP, Righteous Fury becomes something that is always positive, while Moonkin Form still has negatives.
This isn't an iron-clad 100% rule. For example, Auras blur the lines a bit. You could make Auras work like Blessings fairly easily. But in general, Blizzard likes making effects with both a positive and a negative into stances or forms. But effects which are purely positive show up as buffs with a duration. Righteous Fury is a purely positive buff, always increasing threat, so it fits closer to the buff model.