No, it's just that a bunch of people who are neurotic to start with and then begin to feel anxious and desperate as they hear all of the success stories reported here feel compelled to reinforce their worst instincts by insisting on engaging in conduct (LOIs pre-II) that is repeatedly reported to be ineffective, pretty consistently, by the adcom insiders who are kind enough to participate here.
There are no definitive reports of success here because they don't work, and it's just common sense.
IIs are granted based on whether an entire application looks attractive to a school, not based on how desperate an applicant is for an interview. Period. If you haven't made a compelling case for an interview through your primary and secondary (and whatever substantive updates you might have submitted), you have zero chance of impressing anyone by begging in January.
The fact that I am somewhat knowledgeable, and becoming more so every day, while having the objectivity of someone not personally involved in the cycle, should actually make my perspective more valuable, rather than less so, because I am not yet another desperate applicant looking for a justification to do something that couldn't possibly be effective.
I actually agree that, if they make someone feel better, they should absolutely send them, and nobody needs permission from SDN to do so. On the other hand, at least two adcoms have stated that the majority of them are poorly written, they never help (pre-II -- letters late cycle for people on WLs are an entirely different story), and they can actually hurt if a particularly poorly written or desperate sounding one happens to cross the desk of someone who was actually considering granting an II before the LOI was read.