<p>My take is simple. If it adds new information that cannot be garnered elsewhere, then it is useful. </p>
<p>Take for example, a homeschool family. What if the applicant has a sibling in a #1 school. Colleges want more info about homeschoolers because standards are all different. That sibling could give insight into that applicant that no one else can, since they were peers. </p>
<p>And it really helps if the homeschooled sibling is excelling at that top school and the applicant is following the same curriculum. I would think such a recommendation would be useful in showing the success of the homeschool curriculum, especially if their SAT scores are similar too. </p>
<p>OK, I just read the link above about Yale. They state this: “Please do not solicit this additional letter unless you feel it will add substantially to your application.” I guess the subjective part is to determine substantial. But just use the reasonable person test and you should be fine. If substantial then send. If not, then do not. </p>