Why is it recommeded that short term service modifications are excluded from GTFS?

The GTFS specification reference says, "If a service modification will go into effect in 7 days or fewer, this service change should be expressed through a GTFS-realtime feed (service advisories or trip updates) rather than static GTFS dataset."

I was wondering what was the justification for that is? It seems to me that including modifications, even if just effects one stop on one trip that you know in advance, would be useful to both trip planners and arrival predictions.

The only reason I can think of is that people using trip planners might wrongly assume that is how the route normally operates. This could be addressed by added an indicator that it is modified service.