I also think Mab wants a 'handle' on Lara, even disregarding any nefarious desires in Lara's direction. Lara is a rising star, after decades of the WCV's being a passive player. Mab wants to be in on the ground before they become a power that is too big to handle, and Harry's just the guy to keep them from going haywire (remember that other vampire court that got uppity around him?). And if Lara truly is fond of Harry, he also solidifies Mab's alliance.