Just to continue with the posts above, the intent was that the A321s would fly transcons and other high-yeild high demand flights while the 752s, which now have much smaller F class cabins (I think that the US 752s only have 8 seats upfront), would fly lower yeilding routes such as flights to Florida.
As pointed out, some A321s have remained on lower-yeilding routes to Florida and possibly the Caribbean....the allocation system is not perfect. This is one of the problems of an airline having different strategies for different markets and then trying to tailor specific aircraft to certain types of markets, the wrong aircraft with the wrong type of configuration will at times end up flying the wrong route....the situation is similiar to when AA
eliminated MRTC on its 757s and A300s, when a 757 would due a BOS
flight (for example), business pax were treated to the less comfortable aircraft.