Even if pseudonymity is as required for dissent as you say (I doubt it), you can easily build it on top of anonymity with, e.g., PGP.
Why? What would an effective piece of anonymous dissent look like?
There are some famous single work dissent publications though. What about the The Gulag Archipelago?
Comex's point about building verifiable pseudonymity on top of anonymity is also well taken.
Later pieces were strengthened by the reputation built up by the early ones... but how did people know they were from the same author(s)? Just because it said so? Well, anyone could say so. Does that mean it's anonymous, or does the fact that others are unlikely to borrow the pseudonym mean it's effectively pseudonymous?
Today you could just publish a public key with the first article and sign every article thereafter with the matching private key.
Any example of NSA Project BULLRUN success involving a U.S. security product that Americans or our infrastructure depend upon.
Watergate.
The Pentagon Papers.
The leakers in drone division that revealed people are killed based on metadata and possession of cellphones.
The NDA's and legal warnings behind Stingray's.
Any secret conversations between Goldman Sachs and U.S. government before government handed them up to a trillion plus criminal immunity.
Many conversations in liability and risk management discussions in the food, drug, automotive, and defence industries industries.
Conversations between lobbyists and politicians involving all industries.
I'd say these would be a start on protecting Americans from abuse, saving them tons of money, and keeping many from being killed under false pretences. Anonymity helped in many of these cases and would've helped in others. Need more? This country has so much evil that I could probably go on and on.
Note: Upvoted you as before just to make sure people see the examples and remember how important privacy/anonymity is in opposition of corrupt government. ;)