Agreed. This seems great, but ditching standard form elements for a bunch of divs and spans generally ends up an accessibility nightmare.
Backbone.UI.Button = an anchor tag with a span inside rather than a button tag.
Backbone.UI.Checkbox = an anchor containing some divs rather than an input[type=checkbox].
Radio group and Pulldown give similar output.
For me this is a 100% deal breaker. I can understand the desire to dress up checkboxes which doesn't work great as native controls but a lot should be doable just by wrapping it in some custom stuff.
Without form elements it has not compatibility with existing solutions, you can't use this to add some interactive components to an existing form and expect to native-submit that form.
Backbone.UI.Button = an anchor tag with a span inside rather than a button tag. Backbone.UI.Checkbox = an anchor containing some divs rather than an input[type=checkbox]. Radio group and Pulldown give similar output.