Just because not every place needs structured logging doesn't mean that nobody needs it. And those places that do need it do benefit from journal format that can handle it and provide search facilities on top of it.
The *potential* to generate and find structured messages easily is valuable. There are immediate use cases, from usage statistics on services to automatically showing latest messages from log if daemon fails to start---it's not just more work for programmers for no good reason.
We wrote something like this at work because we needed structured log stream: events our users did, and variable messagetype-specific metadata attached to every log message. It made life simpler for programmers, helpdesk and the beancounters. From this, I would predict: the people who want structured messages are probably more than willing to peek into code to add them, because the alternative is continuing the current regex parsing regime, which is inelegant and error prone.