Why is zeitgeist datahub still installed?

[gold_finger]

It's not really a dependency, but something pushed out with certain applications.

Yes, your wording of that is more correct.  In above test case, zeitgeist is definitely not needed for gedit to function properly -- so it's not a "dependency".  I'm just going to guess it's installed because of default settings for apt-get install telling it what other categories of things qualify for auto installation along with the desired package.  (Eg. There are "dependencies", "recommended packages", "suggested packages", etc.  Maybe apt-get is set to auto include all "recommended packages" and zeitgeist falls under that category for gedit.  Just guessing on this here.)