Delta often does "slot swaps" with us (Comair) in JFK so they can get a mainline aircraft in on time at the expense of us being late. If ATC is forcing cancellations, they will cancel us so they can free up slots for mainline aircraft. This happens quite often, especially on bad weather days when traffic starts clogging up the system. Hence, we are often at the bottom of on time performance.

So it is practiced, just not sure how often outside of JFK.