> Like in your wiki example, if A falls, now you don't have A B C D E F... well, the whole alphabet is gone :)
Its far more likely for random individuals of a webring to disappear in my experience.
Ex: A could be the starting point of the webring. When C disappears, then D, E, and F are inaccessible. (A -> B -> Broken page).
A wiki is a single point of failure: you make A more reliable so that everyone has the information. A webring is an infinite number of single points of failure: a single failure (broken link / 404, etc. etc.) messes the entire ring up. Its horrible.