I understand that they can, and sometimes may, force you to divulge your key, but it's not a consistent thing in the United States. There are judgments that require the divulgence, and judgments that claim such forced divulgence is precluded by the Fifth Amendment.
In any case, you buy yourself a very large amount of time if you don't just leave all your communiques out for them to read in plaintext. If you're encrypted, you have a lot of due process to exhaust before they ultimatley MAY order you to divulge the key. They have to go through your lawyer. He can file motions and appeals. The accused may incidentally forget the password. There are a lot of options. With plaintext, there are no options; there is only the government with all your content, and you with no control over anything.
The point is really that they could just change their mind. It's already obvious that the gov't will go to great lengths to violate privacy and what we assumed were constitutional protections, so why would they respect your 5th amendment right and not just demand keys in the future?
In any case, you buy yourself a very large amount of time if you don't just leave all your communiques out for them to read in plaintext. If you're encrypted, you have a lot of due process to exhaust before they ultimatley MAY order you to divulge the key. They have to go through your lawyer. He can file motions and appeals. The accused may incidentally forget the password. There are a lot of options. With plaintext, there are no options; there is only the government with all your content, and you with no control over anything.
See this here, with reference to the conflicting case law in the U.S.: https://en.wikipedia.org/wiki/Key_disclosure_law#United_Stat...