At any station, especially a hub, IRROPS can happen at any time. I'm sure agents don't want to get bogged down working on someone's future reservation and then have 10 people show up because a plane went MX. A policy I'm sure you would want to be in place if you were one of the 10 people and you needed to get moved to the next flight that departs in 30 minutes but you had to wait because someone was working out how to route through Outer Mongolia in order to get to HKG 6 months from now.