Dekamaster is the guy who was, if I remember correctly, HJU's first head "tech guy" (i.e. he was the one in charge of all the technical aspects of the board, and in general, the guy who made everything worked). He was also the guy who made and set up many of the skins that HJU has had/used. Anyway, the "tech guy" job has now been passed on to Web Wizard but obviously, Dekamaster's voice still holds a lot of weight here when it comes to the technical side of keeping HJU running. If he says that you should have fewer problems and that in general, it's better to use http://forums.henshinjustice.net, then it's probably a good idea to do so. In this specific case I think it's largely because the cookies generated by http://forums.henshinjustice.net are more stable/cause fewer problems than the ones from using henshinjustice.net/forums.
It's like if your doctor tells you that you should exercise more to lessen the risk of getting a heart attack. While you don't "have to" listen and/or follow your doctor's advice...given the alternative, well you get the idea.