Leaving the packs off for takeoff won't allow you to prepressurize your aircraft.. and your passengers' ears may suffer from the uncomfortable pressure "bump" at rotation/takeoff. Modern jetliner cabins are normally prepressurized to a pressure altitude slightly below that of the airfield during the early stages of the takeoff roll (to reduce the bump effect).
Rgds.
Q.
[ 08 November 2001: Message edited by: QAVION ]