One point Anna brought up was the link between the index of time and the act of writing. In newspapers or encyclopedias, the content of the writing is linked to the point of time during which it was written. Any adjustments that are made to the text are printed in a later edition of the text, but the old text still remains. Thus the new version and the old version of the text tell us something about what happened in between. Maybe there was a mistake in the text, or more importantly perhaps there was a shift in society that made the adjustment necessary. Dictionaries are modified every year, and it's not because the old version was wrong, but because the meaning and usage of words changes gradually with the passage of time. Being able to see these changes is undoubtedly useful information.
The same is true for history. A significant portion of historical academic writing consists of rehashing and reinterpreting prior historical writing, and trying to untangle the "truth" from whatever biases the author might have been subject to during the time period in which he/she was writing. This article for example attempts to vindicate Ulysses S. Grant from culturally biased historians. However, who is the author of this article to say that he himself is not also tainted by cultural, political, and structural factors that led him to this interpretation of Grant's actions? It is precisely here where, as Gitelman puts it, history is "another victim of postmodernism"
But back to Wikipedia. When someone modifies a wikipedia page, (to my knowledge) the earlier version of the entry no longer exists. Neither is there a visible time stamp next to portions of the page, which may have been modified at different times. Thus when interpreting things that we see on Wikipedia, we are not able to contextualize the content of the page in any meaningful way (not to mention that we don't know who the author is). This is weird. Not weird because they are "intensely labored points of contact, but rather that they are treated as if they are not." I wonder, as we become more and more accustomed to getting information from databases like Wikipedia, if the trajectory of the meaning of terms, events, and people will be succumb to such erasure.