I'm an average height fat guy.
When the mirrors are on the handlebars I need to move them around to see behind me. So I appreciate the fact that I can move them around when they are mounted to the handlebars. But, when I'm riding I can't just move the handlebars around willy nilly like that so I'm basically blind behind myself unless I do yoga instead.
When the mirrors are mounted on the frame in front of and beneath the handlebars, I don't need to move them to see behind me. So, I don't miss the fact that I can move them around while backing up at low speed. And when I'm riding I can also see behind me, so I don't miss having to do yoga to glance behind me. Basically, I can just see behind myself properly so all of the workarounds from standard bar-mounts don't apply.
In other words, this reasoning is circular. The mode that makes it easy to move them also requires you to move them. The other doesn't and doesn't.
[Important qualifier - I am basing this on experience with the BMW R1200RT implementation of this mirror placement. I have no idea if it applies to Zero's very similar placement, but I'm hopeful...]