There is your answer, someone who already is a wizard when he or she becomes a host is more apt to be a partner than someone who is a mere sock puppet.. I went and reread the part of Battle Ground where Harry takes over the castle and talks about the runes with Molly. He says that Marcone never did much with them because he had no clue about what they meant and Namshiel either couldn't or didn't teach him... Which says Marcone never was a wizard, if he were he'd get the importance of them, and be most interested in Namshiel translating them for him.