Way back in the day (pre-9/11, I think summer of 2000 or so), DL did have a BDL-JFK flight on a 767-300. Was done for international connections and to ease up on gate space at JFK.

LGA is definitely a slot squat though (as are PVD, ALB and to a lesser extent ORH). That being said, it does make DL more of an option for connections, notably in the Virginia/North Carolina area where ATL/DTW are a bit out of the way.