Posting on SDN is good practice for the writing sample on the MCAT. The rapid feedback on grammar, syntax and spelling, as well as analysis of logic (or lack thereof) all militate favorably to a high WS score.
Too much time spent on SND will work against this. Time should be better spent on studying for PS, VS or even the dreaded BS sections (although many have said that all the sections are BS). The attraction of no more MCAT studying, for whatever reason, is powerful.
A careful balance, then, is key. It is important to practice the WS and all sections of the MCAT to achieve the SDN MCAT modal score of 40-S. It is important to maintain one's sanity at the same time, and SDN provides a conveninet relief outlet.