I recently read Peace Talks/Battle Ground and it really struck me how interested in and involved Mab appears to be with Marcone.
After just a moment of wondering why an idea struck me - I think Mab sees Marcone as a candidate to be the next protector of the Outer Gates. Not a Fairy Queen or the Gatekeeper, but rather the monarch of the new order that will replace the Faerie Courts after the BAT. Marcone is ruthless but he doesn't take sadistic pleasure in being cruel and it can't be disputed that he uses his power to protect innocents. Mab may very well see Marcone as a mortal embodiment of herself and is guiding him to create his own Mantel and displace the Faeries when their time as the significant power ends.
Tragically, Harry could become the counterbalance to Marcone and be forced to deal with him for the next 666 years. (Although it isn't clear whether the protector always has a counterbalance or whether that is just a feature of the Faerie Courts, and the White God/Lucifer).
edit: In fact, Marcone and Harry already have names picked out for their potential Mantles - Baron of Chicago v. Wizard of Chicago.
