VH ABC, Boeing doesn't recommend NOT to arm VNAV. The objective of the bulletin is: " This bulletin informs 737NG flight crews of the need to revise the windshear escape maneuver if VNAV has been armed on the ground for takeoff (FMC Update 10.8 and 10.8A)."
Now if you operate mixed FMC's (10.7 and 10.8A), there could be a technical reason NOT to arm VNAV on the ground: You don't want your crews to arm VNAV on 10.7 and you don't want them to follow an AFDS windshear maneuver with VNAV engaged on 10.8A. To escape the consequences of these errors, fleet managers could prohibit the use of VNAV on the ground until softwares issues are solved (not the case in my Company).