Arming VNAV on the ground is not permitted with FMC Update 10.7 or earlier.
With U10.8 or 10.8A installed, if unstable weather conditions are present prior to takeoff, consider not arming VNAV for takeoff.
Direct quote from the Boeing bulletin under OPERATING INSTRUCTIONS.
Mooseknuckle, you originally asked the question as to a technical reason why you would NOT arm VNAV for takeoff. Isn't Boeing giving you one in this bulletin?