Speed is generally a more valuable asset to a winger than a center in most NHL systems. For instance, look how much more AA uses his speed as a winger. Very few centers consistently get breakaways, wingers do pretty regularly. AA certainly had more breakaway goals off the wing. Look at some of the better Wings centers just in recent memory. None of Datsyuk, Zetterberg, Glendening, or Nielsen are especially fast. Filppula is probably the best skating center we have had in the modern era until Larkin came along. And that's just our team. None of Toews, Getzlaf, Spezza, Malkin, Tavares, Thornton, Johansen, or Pavelski seem to have suffered as a result of not being especially fast skaters.
And all of that is a little beside the point because Rasmussen is actually a pretty good skater, especially for his size. I think his skating is probably roughly as good as a many of the non Wings mentioned above. Hardly a liability.