That's an important distinction. In advocacy, you start already knowing the conclusion you want to reach, and you try as hard as you can to convince the audience of it, the way a lawyer or a politician would. Whereas an essay, as the name implies, starts with a question, and from there you try to follow the truth wherever it leads.
I wrote about this in "The Age of the Essay" (http://paulgraham.com/essay.html).
There is not a mathematically sharp distinction between the two approaches, of course. A lawyer or politician will use the truth when it suits his purpose. And an essayist trying to follow the truth will inevitably have biases that cause him not to follow the truth as far down some roads as others. But though the two approaches form a continuum, and writers sometimes drift along it in the middle of a given piece, the distinction is a meaningful one.
For example, you write: "As anyone who has worked for the government knows, the important thing is not to make the right choices, but to make choices that can be justified later if they fail."
What evidence do you have of this? What about agencies like DARPA? Your statement could also apply to employees of large corporations.
You are doing the same thing that Joe Stigliz did in the article (without his academic ethos)