I also implemented the current output format (as "olde", though that's a little perjorative so I don't mind if you rename it "mozilla" or some such). If I got it right then no core functionality should break - but obviously it needs testing. The implementation is very simple, it was just a refactoring exercise really.
I don't really know anything about junit. I suspect that SoftwareTesting wishlist was written quite a while ago, I'm not sure if this is something that's actually useful to Mozilla. The current output format, while totally arbitrary, is useful to us because we've made it (mostly) consistent across all of our unittest harnesses.
The current output is fine for testing solely within the Mozilla system. But my work was prompted by the need to integrate testing of a Thunderbird plugin with the rest of my site's development (ie. with Hudson in my case)... Other people will have this problem - are there currently *any* tools to help plugin developers do regression testing? - and using "public" standards for test output formats wins you/us the ability to integrate with other (massively used and developed) tools. There's no plugin to integrate MATOF "Mozilla arbitrary test output format" with Hudson or any other tool as far as I know... Simply taking the code I've written, with no other work, and including it in the core will help anyone wanting to integrate with external (to Mozilla) test tools. In the long run perhaps it can be seen there's a one-to-one mapping between MATOF and TAP.