For certain schools, and not for others. Either way, as Carmiche also says, the use of that website appears to have dropped off (at least somewhat). Not to mention that in my opinion, and experience, it is not very user friendly. I'm not trying to be unappreciative of the SDN people for maintaining the site (far from it), but features could most certainly be added to make it easier to use. I have an account myself there, and I have updated my information as it has changed. However, when looking at some schools there might be, for example, one person from last application cycle, and then a ton of people from 5 or more application cycles back. That might give good historical data for those cycles, but it doesn't do a lot to give data about more recent cycles (and certainly not the current one). Again, especially at certain schools.
Also, MDApplicants has the "University of South Carolina" listed as one school. However, there are actually two medical schools in South Carolina that begin with "University of South Carolina": Columbia and Greenville. Both of these schools have separate applications, admissions committees, administration, etc. In other words, they aren't just two campuses of the same school. But on MDApplicants you can only list "University of South Carolina", and you can only do it once. So, students who applied to Greenville will use this same entry as will those who applied to Columbia, but users of MDApplicants won't know which school the data is referring to unless, like me, the applicant specific lists additional information in their entry. Then, again like me, you have applicants who applied to both, but don't have the same outcome at each: I was rejected at USC-Columbia, but am still under consideration, at present, at USC-Greenville. There are probably other examples of this too that I am just not personally aware of.
So, in my opinion, it does serve a purpose, but it also has its limitations. Furthermore, adding a thread like this in no way diminishes from that site (nor does it mean that a student can only use one or the other, they can use both).