Eminem brings different fans than Wayne does, that's why that No Love collab was important for Eminem to have to drive sales of his album and soften him up to the group of suburban people that think his topics are radical. I do see you point of Wayne being on top in regards to popularity, but it's like having any other feature Tech has had.
He got Bumpy to appeal to New York, more specifically that portion of the East Coast.
He got Scarface and Paul Wall to appeal to Texas, aka the mecca of Southern rap.
He wants Em, not necessarily for Michigan, but to garner interest from one of the 3 top dogs fanbases; he wants Em because he IS American mainstream hip-hop, and more specifically he wants Em because he knows the guy will compete with him lyrically in a way that nobody else has really tried to do.