Yep, being stealthy is the key. Even if the army gets routed, the scout should be safe (considering that it doesn't participate in the combat). This can though backfire in the way that if your army routs, and you need the gems for the routed forces next turn for something, they are still in the province you were moving to. Also, if your army gets attacked for some reason in the province they were in, the scout moves on but the army might not. You can also store magic gems (not blood slaves though) on another mage/commander in the battlefield that doesn't have the magic skill associated with the gems - they won't use them as far as I know, since they effectively can't cast anything from the path.
But generally, about gem usage. It can be used to great effect. Gems can be used by mages to:
1 gem -> boost path by one
x gems -> pay the spellcost
let's have an example:
Shadow Blast consumes one magic death gem to cast it. You give the d2 mage 2 gems and script it to cast Shadow Blast x2. The mage gets all "YAY GEMS" in battle, and uses the first gem to boost it's death path temporarily to 3 for the first shadow blast spell to reduce the fatigue and then consumes the second gem for the spell itself. The mage then proceeds to it's next order, which would be shadow blast again, but it has no gems left, because it burned it off to the first spell, so it probably goes and promptly summons a few skeletons and goes unconscious from the fatigue, or something as useful as that. What I'm trying to say is to usually give either few extra gems to the mage for the fight if you can afford it, or simply use very few gem consuming spells per scripting.