I agree about the "hot" goods argument, however if Marcone thinks that somehow he can gain leverage over Harry because of it, he is sadly mistaken or rather doesn't know Harry as well as he thinks he does. Whether anyone can make use of the Eye again is open to debate. I don't think on one hand Jim would have made such a big deal out of it if it wasn't going to be used again. Then again does it take hate to use it or does it corrupt the one using it to hate? And more importantly maybe, would the user have to give up an eye not unlike Rashid to wield it?
Mab's comment about love/hate not being opposites (rather hate/logic being opposites) may be a loophole that could be used late in the series. Say, Harry's love for Maggie allowing him to use it. Idk if that's likely, though. Alternatively, if the Outer Gates fall in the BAT, Harry could grant an ally the Eye (Odin?) to help defend the breach. Regardless, we've probably got several books until the Eye becomes a front burner issue again.