I have a DP Quad + DP Dual and a DP34 switch, it doesn't make any difference if I check it or don't check it. So I guess you are right Iceberg, it's more likely for Legacy configurations.
I was just curious because in that new Dish Support site that Scott posted a few days ago, they have the interactive menus for all the receivers. When I went through the 501 menus to "check switch", it's not checked but when I go through the 311 menus it's checked.