I have noticed by listening to my scanner that in busy airports with multiple departure runways the runway assignment doesn't come until at a relatively late departure stage concerning takeoff planning.
The best example I can think of is Chicago O'Hare, this would be one real scenario: ATIS reports "departing runways 04L, 09L, 14L, 32L, 32L at T10". That's 5 different takeoff options. When listening to clearance delivery, a departure procedure is assigned, followed by a flight route and/or "as filed", the initial altitude, flight level to expect, and squawk, but NO runway assignment yet. Then it's ramp control for pushback clearance. No runway assignment. Ground metering to report "ready to taxi". No word on what runway yet. It's not until ground control when a runway is assigned, many times resulting in "Request 32L (longer runway), we're pretty heavy today"...
Why are departure runways not assigned earlier, for example with clearance delivery? Doesn't this require a little bit of unnecessary guessing by the crew? How about performance issues? If you're assuming ATC will assign a certain runway and in the end it turns out it's something else, is it fast and easy to reload FMS takeoff runway data and performance or do these new calculations take an extra while?
Airline pilots replies are really appreciated, but of course all replies are more than welcome.