Very simply, it is a minor league deal, the guy has some talent and the ink/PR from it will be well worth the invite. Now I know Nomo is really not as hyped or touted as he once was, but he does have many supporters over in Asia. With the Sox having Iguchi and giving Nomo a shot, it can't hurt their image over there. Combined with all of the international talent on the roster and some of the marketing they have used, it seems pretty clear they are trying to gain support in foreign countries to attract players.
From a baseball perspective depth at SP is great to have, great for trading midseason as well. Can never have too much starting pitching, never.