I spoke to my professor at school (who is a pastry chef-I'm in pastry school)and she claims that there is no difference if it is added or not.
she claims that wilton says to add the powder as a marketing strategy. (maybe people will buy more cans of the half empty meringue powder)
(my professor does not like wilton to much!!!)
But, all I can say is that when I tried it, it did not work.