This is very insightful.
Wikis are for text that people want to read and change, regardless of its source. This makes them ideal for Wikipedia but inherently out of tune with the big-corporate environment, a large crowd in which people prioritize based on who is talking to them. A boilerplate memo from the generic HR-department address can be skimmed very quickly; an email from HR addressed solely to you might need to be read; an email from the HR rep who has been assigned to your team might need a more careful reading and perhaps an acknowledgement; even a boilerplate email from HR might need careful attention if the author is someone you befriended at the company Christmas party.
You don't want an automated alert every time your boss's assistant reorders the columns on the weekly report. Ideally, your boss will only call your attention to the report when it is important for you to read it. That's part of the boss's job -- to protect your attention so that you can spend your time being useful instead of rooting through irrelevant wiki documents! And how is the boss going to direct your attention? Email. And while she's sending email, what's wrong with just pasting the information itself right into the message and saving the recipient some additional clicking?
Put all that stuff on the wiki and file off the TOs and FROMs and datestamps and you might as well just label it BOILERPLATE, because nobody's going to have the time to sort through it all. Even a genius super-librarian can only help so much, because the social context of email (its timing; the number and identity of the CCs, the history embedded in all the quoted emails that extend far down the page, etc.) is hard for a librarian to capture, let alone convey in wiki form. And, no, the version history does not help much. (Deriving people's motivations by reading their diffs is a techie art, not a liberal art. And emails carry a semi-reliable record of who has read the message as well as who has helped to write it.)
(A minor note: You left out Powerpoint in your description of the "enterprise wiki". It's pretty important, and for more than just stupid effects-laden bullet points. I worked in an engineering firm. All of us were capable of understanding wikis. I never tried to set up a wiki, though, because people communicated in charts and graphs, and at the time I couldn't find a wiki whose chart-and-graph workflow was anything short of "excruciating". The state of the art was to mail out arguments that consisted of stacks of graphs with commentary, rendered as Powerpoint docs.)