I am totally not getting the purpose for XForms. This smells like another out-of-control “standard” that promises the world, and ends up delivering mostly bloat.
Traditional HTML Web forms don’t separate the purpose from the presentation of a form. XForms, in contrast, are comprised of separate sections that describe what the form does, and how the form looks. This allows for flexible presentation options, including classic XHTML forms, to be attached to an XML form definition.
So we are going to get automagic rendering that works wonderfully in a big browser window and a tiny cellphone screen? More likely we get something that works well on one but not the other - or on neither. Optimal is not the word to describe this. If you are going to present a user interface on radically different devices, for any sort of decent result you are going to design a different user interface - and this you cannot escape.
Since I am tending more toward populating, updating, and submitting forms as JSON, injecting XML in the form of XForms seems more like a step backwards - or at least sideways.