Fat poured into pan doesn't guarantee the kind of pristine coverage necessary for a good crusty grilled cheese simply because it doesn't get the opportunity to penetrate as far into the bread, only the outer most bit of bread catches a sear and then that sear prevents further fat penetration and also mayonnaise is best IMO because you get the best crisp to fat ratio, you can use much less mayonnaise than butter and still get wonderfully crispy results.