Why would such a figure be a particularly good moral archetype for a fantasy roleplaying game?
Do such entities exist in the related fantasy fiction? Is it fun to play? Does it open up a set of gaming options that might not otherwise exist?
Yes, keepers of balance do exist in fantasy fiction. Several posters in this thread have mentioned examples. As to whether it's fun to play, I'd say that it's more commonly an NPC alignment, but the same applies to Chaotic Evil IMO.
It most definitely opens up a set of gaming options that wouldn't otherwise exist. If you want to preserve the balance, you aren't good or evil, lawful or chaotic. You'll ally yourself with any side that's become the underdog, in order to restore balance. That's most certainly different than what a differently aligned party/character would do.
If you ask me, the "maintain the balance" alignment isn't an alignment at all. It's either a character quirk or some kind of neurosis, but there's no need to hardcode either into an alignment system.
If you ask me, the concept of alignment probably arose from the struggle between Law and Chaos seen in books like Zelazny's Amber series and Moorcock's Elric series. In those books, certain individuals are
aligned with one philosophy or the other, and struggle against opposing aligned individuals. Their struggles are part of a greater cosmic struggle.
In that type of setting (which D&D, with it's Great Wheel of philosophies as existent planes, arguably is) it's not a quirk or neurosis to seek balance between the two. Without those opposing forces, the universe as we know it ceases to exist. Wanting to preserve the known universe doesn't make one crazy, it makes one a hero. However, this is a very peculiar hero in that he's just as likely to ally himself with demons as with angels, dependent on which side is losing.
In all fairness, this is the most legitimate critique, IMO. However, this definition of Neutrality has been part of the game for a long time now. If they're bringing back alignment for the sake of tradition, then there's certainly an argument to be made for the tradition of preserving the balance.
All I'm saying is that there's very little in common between someone dedicated to Neutrality in the sense of maintaining a balance, and someone who chooses not to participate in the "alignment wars". The first is driven by a strong sense of philosophical purpose. You're not likely to bribe him into betraying his ideals. The second is more like a mercenary. There will be some things they're unwilling to stoop to, but beyond that they probably have a price. In a sense, the latter is driven more by practicality than purpose.
Removing preserver of the balance from the alignment mix certainly is an option, however.
Explain to me this. Assuming we want to preserve the keeper of the balance concept, what harm is there in making the distinction?