I don't know whether to applaud or criticize those who create open source software on unstable proprietary platforms, but if nothing else it shows what potential good greater openness would serve users.

Um...have you paid attention to the open source space for the past decade?

Audium/Pidgin/libpurple all were originally created to have open source interfaces to close systems (MSN, AIM, Yahoo Messenger, etc). Most of the OSS work is about reverse engineering and creating open yet compatible tools to use with or in place of closed systems.

If anything, projects like this can help keep Pebble watches alive after the demise of the company.

I don't think we're in disagreement. There is end-user value at least in short term. But when efforts are expended to keep users engaged with and thus financially supportive of closed systems instead of building alternatives isn't that folly?

None of the closed systems you mentioned exist in any meaningful form today and it's hard to imagine that a substantial number of Pidgin/Adium users began to sing the praises of XMPP rather than adopt another closed messaging system.