Let your wise lord and master, Cornuthaum, speak on this issue, that you may be enlightened:
Admins: Multiples
You may look at me aghast, now, and say "Cornuthaum, raptorious overlord, are you mad?", to which my reply would be to savagely devour you and then educate your fellows: Have we not seen what having only one Admin-tier power can do to a forum, if that admin savagely betrays his forum by just leaving and not coming back? No. Two, at least, ideally three are needed.
Mods: Multiples
Once again you may look at me aghast, and ask "Cornuthaum, raptorious overlord, are you unsound of mind?", to which my reply would be to savagely devour you and educate your clone once it is released from its tank. One mod cannot hope to police any amount of forum, especially one as inherently factional as a Fanfiction Forum. THE Fanfiction Forum, to be precise. It is unavoidable that a mod does not care much about Fandom [x] and much more about fandom [y] and a bit about [z] while another mod would be all over [x] and [ψ] and [ä] instead. Multiple mods serve to ensure that there will be always someone to take care of problems, no matter the fandom.
Modly Powers: High
You, minions, might now shake your fists at me and claim "Cornuthaum, raptorious overlord, you want a fascist police state!", to which my reply would be to hit you on the head, then savagely devour you and educate the huddling masses that remain.
A mod must have the powers to make their word heard and heeded. Castigating the unworthy rabble with their modly powers is, alas, usually the only way to make this clearly understood. If a mod drops in to say X, and then the infractions against the rules of good behavior go on because nobody fears the mod's word, something has gone wrong. Look at SpaceBattles and RPG.net - they're doing it right.
Rotation: Fuck no!
You, dear minions, might look at me in confusion now, my reply to which would be to grant you a ten minute pause from your work in the salt mines that you may listen to my educational words.
A mod's work is akin to herding cats, if the cats were tricksy, sneaky and altogether more depraved than even real cats are. It is a job that requires focus, dedication and wanting to do this. What more, the proper execution of modly duties requires that they build up a certain experience with the tools of their station: as such, rotating them out would counter both of these points, which is therefore why rotation is fucking stupid and should not be implemented.