I really don't think it makes much of a difference in admissions...people on here tend to assume things are required just because all the SDN-types are doing them, like letters of intent and thank-you notes... I really can't imagine an admissions committee sitting around going "this candidate is pretty good, but he didn't send that letter of intent we never mentioned and if he wasn't on SDN he'd never have even heard the concept of."
this is just a silly escalating arms race... next year someone on SDN will post that they sent a research results portfolio, or a Christmas card to the admissions office, or an unsolicited essay on why they're a good fit for the school, or a proposed research project they will undertake with one of the schools' faculty members, or will physically visit the admissions office outside of an interview day to 'demonstrate interest,' or anything else you can think up... and then everyone will start doing it. and the admissions committees will be like "uh guys, we didn't ask for any of this, there's a reason we have a standardized admissions procedure"