Since the last thread is now locked..
Addons that provide you with more information about your character (i.e. your damage output, cooldowns, resources, debuffs) should be allowed. Those that provide you with more information about your opponent's character - information that is not otherwise available in the game - should not be allowed.
Addons can improve one's gameplay, but should by no means be allowed to steal from one's opponent the element of surprise. Pretending to be out of mana/stamina, only to lure in your opponent and turn on him with your remaining resources, is a skilled play... a play that overextending addons would invalidate.
This is my opinion, anyways. I think this debate often boils down to whether or not addons should solely provide increased enjoyability of gameplay, or a competitive advantage. Realistically, addons will always exist to help one player defeat another, I say the question is to what extent to these addons provide an advantage.
Some of these addons that extract information from other players' APIs are akin, in my view, to Xray goggles.
To those of you who say that addons do not give an advantage ("the more skilled player will win"..."addons don't win fights"..."), of course skill can't be replaced by addons - but it can be augmented.