Basically it is so D3 athletes can participate in signing day if their school has one. It gives the student something to sign.
I know several Baseball players who have already "committed" to a D3 school. In my opinion this is not a good idea. The student may have an idea what they are going to pay for he school, but I have seen more than one player commit only to change his mind once they received the financial package. If you have already committed to a D3 school what leverage do you have in negotiating with admissions and the financial aid office.
I think it is far better to keep your options open. From the article, since the letter is non binding, other schools can still continue to recruit you. But then whats the point of signing? How many coaches are going to say, "yeah, it is not binding but you committed". In reality their is not much they can do, but is their a point to signing a letter that in reality means nothing?If a High School chooses they can still celebrate the D3 commitments, if they chose to without this.
The more I think about it the more I think it might be the first step down a slippery slope.