I haven't seen any evidence of legacy US 321s being "subfleeted" and older ones relegated to any particular market. They all appear to go whenever 321s are currently assigned. For example, I've flown some of the oldest and some of the newest on PHL-SFO in the past few months and one of them did come in from CUN.

If you encounter one in bad shape, I would certainly complain (and take pics). My experience with the 321 fleet is that most of them are in pretty good shape, even though the oldest are already 15 years old (some of which have had new seat covers and other interior touch ups, but not a full refit), with some of the newest months old and legacy AA-config 321 deliveries continuing.