Well, `dpkg.log` has entries for when packages are upgraded to newer versions. For example, I have
2014-10-03 14:43:03 status installed pulseaudio:amd64 1:4.0-0ubuntu21
even though the pulseaudio package was installed before that, at an earlier version. If I were to parse dpkg.log to dump package names from "install" lines and purge them, I'd end up removing pulseaudio and a number of other highly essential packages.
(Sorry, my original example was bad because it was actually from the original system installation.)