I participate in developing anti-censorship tools. Chinese are a significant population, and it has some overlap with activists there. In practice, identity settles at "who controls this email address".
Self-signed PGP is very occasionally used to prove continuity across channels or addresses. Cross-signed basically never.
For Debian-style applications that are 100% about openness and 0% about secrecy, sure.
But if you want to secure communications between pro-democracy activists in China, or you're a Snowden-like whistleblower wanting to securely communicate with journalists - y'all probably don't want to be vouching for one another's keys.