CO (and the other airlines) want maximal operational flexibility. What I can't figure out is, you have a situation where you know what planes are going where, and how long it will take them to prepare to go. 3 hour + tarmac delays don't just "happen". At the most you need 10 minutes worth of departures ready to go, 20 minutes worth ready to close, and the rest can relax until ATC gives the go.