There's also a big difference in impact between whether you're flying nonstop or making a connection. A few years back was "The Summer From Hell" at SFO, my base for most of my flying at the time, and probably about 50% of my flights in/out of SFO were IRROPS. Thankfully most of my trips were short, non-stop flights so the impact was not severe. Hour and a half or two hour delay? Annoying, but at least I'm not stranded somewhere 1000 miles from both home and destination because of a missed connection. And with short nonstops there's rarely any point asking to be assigned to another flight.... Usually the delayed nonstop is still the fastest way to get there!