Conditions on reporting turn reporters ever so slightly from press to publicist. And at that point it's like the cliché about being willing to sleep with someone for millions -- the character has been established and the only thing left is negotiating the price.
Even though the article says it's not done, you have to imagine that some reporters will agree to change a conversation into an "off-the-record" one in exchange for access or a scoop. News orgs live and die by the access they get and if some reporters are willing to prostitute themselves to get it, it puts enormous pressure on the rest.
Conditions on reporting should be treated like doping in sports.
Edit: there is one area in which my framework needs a clarifying remark. A reporter can always voluntarily refuse to identify a source or otherwise protect it and the source can always use electronic or even snail mail methods to preserve their anonymity. My comments above are not intended to mean that reporters must always report every detail of every interaction, only that agreeing to conditions in advance can compromise their integrity and pervert the role of the press generally.
2nd edit: I'm convinced there are areas where granting anonymity is absolutely essential. My above position was clearly a knee-jerk and not well grounded in reality. Thanks HN.