Ah, true, I disregarded necrons here... I still say that 6+ save against lance fire is superior option then
hitting on 5+ makes all 'critical' effects more likely on lock on - necron whip, phantom lance, Admech with weapon upgrade and so on. Every lance weapon with different effects on various rolls. While hitting on 5+ changes these weapons' internal dynamic, 6+ save, while having similar effectiveness leaves original internal balance of the weapon in peace. True, it's one more roll, but at least every result is still possible. (5+ eliminates DE inflicting single hit, -1 to ht eliminates weapons with extra effect on 6.)