s-j-upload is currently designed to export data from clients to a centralised s-j-remote instance.
Supporting alternate output formats such as JSON will enable s-j-upload to export directly to a wide range of log consolidators. Examples are Logstash, Splunk, Graylog2 etc on the Open Source side, with a wide range of proprietary corporate applications also out there.
While s-j-remote is build as the default recipient of data from s-j-upload, not every user will be interested in consolidating their logs in this way.
Perhaps this could be packaged separately to the core systemd applications so user would have to specifically install this package in order to alter the output from s-j-upload?
We already have json formatting implemented in journalctl. So it would be a matter of hooking that up in s-j-upload, probably not too much work. I'd be happy to take a patch to do that, but I'm unlikely to implement this myself. Primary reason is that I don't run anything like logstash/splunk/graylog2, so I wouldn't even know how to test if the thing is communicating properly.