I am guessing you might be joking. Delta has long called the MD88 the accumulator fleet. They could be put in and out of service at virtually no cost and as such be used to quickly adjust capacity.
If you have a aircraft with a lease payment you still have to make that payment if you park it. If you lease a aircraft for 10 dollars a month to keep things simple would you park it or keep flying it even though it earns only 9 dollars a month. A dollar a month loss or a 10 dollar a month loss? Same thing applies if you own the aircraft with a mortgage payment.