The Gemara raises concerns about the potential for fraud via the document that has been witnessed, if text were added to the document and a new signature were added to confirm it - as if it were all part of the original. And likewise, a practice of signing designed to prevent this kind of effort from working, both in terms of the direction of signing and cutting off the extra room. Plus, what if one signs in his father's name? Or uses a symbol to sign instead of a written name?