As a security layperson my initial reaction was "how can cryptography help with expiring messages, once it's decrypted it's decrypted, that doesn't sound right", but I'm curious if I'm understanding correctly that this is actually two separate features: 1) clients voluntarily respecting "please delete this message at X time" and 2) forward secrecy. And Keybase has tied them together for UX reasons since people tend to not have an intuitive understanding of when they might want to use forward secrecy, but they always want it in the case of exploding messages.
This is not functionality meant to make sure the other party will not have access to the massage after X time anymore, it's just convenience opsec: If you don't want someone to know X after some time, you should never tell him in the first place (he doesn't need to hijack the keybase client, he can simply "remember" the message). Instead this makes it easy to limit paper trace using a nice UX.
This is a good summary. Keybase supports adding new devices to your account at any time, and for that reason people often don't want forward secrecy. (Their message history would be gone on their new phone.) But that isn't a problem with exploding messages.
