Tom,
I am not sure what you are referring to with mixing tcl and markup using ad_form.
I rewrote the admin pages for the simple-survey to survey package using ad_form. I removed all the HTML from the tcl files. I also eliminated several pages, and generally improved the user interface.
I need to repeat the benefit Don mentioned which you get when using ad_form or the form builder directly, inline error messages. This user interface improvement alone makes it worth using the toolkit features to process forms.
