I think the 40-seat CRJ came out of DL's pilot scope clause where the average seat capacity of the RJ fleet had to be a certain number below 50. The 40-seat RJ helped the average seats across the entire DL Connection fleet to be lowered.There was some reason why Delta bought CRJ-200's and installed only 40 seats. The only thing I can remember is that the agreement with the manufacturer prohibits Delta from adding the other 10 seats in the future. Unlike most planes, the bulkhead seats on that aircraft are nice, because you have plenty of legroom.
There should be more baggage capacity on that plane owing to the weight of 10 non-existent passengers being used for baggage (if the limit is MGTOW and not center of gravity or physical space in the hold).