I am glad to hear VTK functionality will be included in the form builder.
I think it is very important to include both client side and server side validation on every form.
The client side validation (spry etc) is nice, because its faster because the form does not need to be submitted for it to show errors.
Anyway, it sounded like you were starting to lean away from server side validation and lean more heavily on client side validation, and I wanted to put my 2 cents in there. If I had a choice of only one of those technologies, I would choose server side every time.