Encylopedia articles have structures. And encyclopedias taken as a whole have structures. They are not simply loose conglomerations. (The means Wikipedia uses to produce an encyclopedia means that in progress it tends to look more like loose conglomeration than other encyclopedias, but that is not the goal). I doubt that many people actually use the Britannica "Propaedia" but the quality of the Britannica is probably due in part to its existence. Conversely, the value of the "yearbooks" that encyclopedia publishers hawk to "keep your encyclopedia up to date" is low because even though the individual articles in it may match those of the rest of the encyclopedia in quality, the "yearbooks" do not fit into the structure of the main encyclopedia. This is not simply a matter of indexing them; successive editions of encyclopedias are _not_ produced by incorporating the yearbook articles into them in alphabetical order. The effect of editing within an article affects the structure of a single argle. The effect of creating or deleting an article affects the structure of the encyclopedia as a whole and is therefore a more significant event than edits within an article.