The reason Taibbi comes up with are sprinkled throughout the article:
Although Assange may not be a traditional journalist in terms of motive, what he does is essentially indistinguishable from what news agencies do, and what happens to him will profoundly impact journalism.It seems to me that most Americans look on the activities of "their" government as crazy and corrupt most of the time, and thus ignore what the government is doing by focusing on their own private lives. The underlying assumption is if they ignore it, it will go away and essentially not impact them. This assumption, I am convinced, is a gross error that will likely be the end of all of us, one way or another.
Reporters regularly publish stolen, hacked and illegally-obtained material. A case that defined such behavior as criminal conspiracy would be devastating. It would have every reporter in the country ripping national security sources out of their rolodexes and tossing them in the trash.
... in terms of what an Assange prosecution would mean for journalism in general. Hate him or not, the potential legal consequences are the same.
... a prosecution that uses the unpopularity of Assange to shut one of the last loopholes in our expanding secrecy bureaucracy. Americans seem not to grasp what might be at stake. Wikileaks briefly opened a window into the uglier side of our society, and if publication of such leaks is criminalized, it probably won’t open again.
There’s already a lot we don’t know about our government’s unsavory clandestine activities on fronts like surveillance and assassination, and such a case would guarantee we’d know even less going forward. Long-term questions are hard to focus on in the age of Trump. But we may look back years from now and realize what a crucial moment this was.