EagleFlyer,
We have a Boeing Bulletin that states:
"Boeing has received reports of inconsistencies associated with the arming of
VNAV prior to takeoff on 737NG airplanes equipped with FMC Update U10.7.
These inconsistencies can occur even on those airplanes with the configuration
specified in Service Bulletin (...) installed, i.e., FMC Update U10.7,
CDS Blockpoint 06 and Collins Autopilot P4.0.
Additionally, VNAV may incorrectly engage rather than arm if an FCC version
earlier than Collins FCC P4.0 or Honeywell -710 is installed. For FCC P4.0 or
Honeywell -710 or later installations, VNAV may engage rather than arm under
certain conditions when the departure is not aligned within 5 degrees of the
runway. With VNAV engaged on the ground, when TOGA is pressed VNAV will
disengage and the MCP IAS/MACH display will open and display 120 knots
(Collins FCC) or 100 knots (Honeywell FCC).
This anomaly exists only with FMC Update U10.7. It will be corrected in a future
software upgrade of the FMC and/or FCC."
Hence the fact that, arming VNAV on ground is prohibited for aircraft equipped with FMC 10.7.
MooseKnuckle,
Mixed Fleet Flying restrictions could be a reason as you state it.