Just as a document is made for reading, forms are made for filling. In a workplace or workflow that requires accessible documents and forms, paper forms or TIF files aren't acceptable because they cannot be made fillable by an assistive technology (AT) user.
Its for this reason that any document used expressly to collect information (ie, a form) must include form-fields or other means of being filled by AT users. The form must provide equal access to its content, and must be fully interactive, with correct tab order, field attributes and so on, in order to conform with the law.
Lets take a look at the Section 508 regulations:
Subpart B - Technical Standards SubSection 1194.21
Software applications and operating systems. (l) When electronic forms are used, the form shall allow people using assistive technology to access the information, field elements, and functionality required for completion and submission of the form, including all directions and cues.
This text (while appearing - unaccountably - in the "software applications" section), clearly states that if you deliver an "electronic form" for "use" it has to be fillable in order to comply with the regulation.
There's no provision for intent. "Fill with a quill" forms are simply not an option in Section 508 or WCAG 2.0 conforming applications, and good riddance to them.
A form is ALWAYS made for filling, ergo, all forms must be AT-usable fillable forms.