It's fairly simple to follow from there, isn't it? If the software you use falls under such definition, then you are (unless handy with advanced debuggers) mostly blind to what it is doing. To claim as much is the very opposite of "closed-mindedness".
Now, according to the credit at least, Stallman didn't write this article (though you are obviously correct in suggesting that his ethos drives the point). But can we really in this PRISM era point to Stallman, whose historical warnings were obviously prescient beyond the basic measure of sense held by the dominant digerati, and dismiss him as 'preachy'?
I'm still of the opinion that nonfree does not equate to nongood. It should be more like "nonfree software is not easy to audit, therefore it is potentially not safe." When people make blanket statements that are overly simplistic, I automatically feel like their logic is compromised. Their emotions or prejudice are getting in the way of accuracy, which is important when making statements about what other people should do.