[narfforc]

I have always seen the Rings as amplifiers, if the person wearing it was of a certain persuasion, he/she could make their inner most desires become greater through the use of a Ring of Power. If we apply this to Saruman, maybe what his Ring did was add power to the Power of his Voice. If we take Narya and add this statement:

Take this Ring, For thy labours and thy cares will be heavy, but in all it will support thee and defend thee from weariness. For this is the Ring of Fire, and herewith, maybe, thou shalt rekindle hearts to the valour of old.....
(The Silmarillion, Of The Rings Of Power).

Then add this statement:

Warm and eager was his spirit (and it was enhanced by the ring Narya), for he was the enemy of Sauron, opposing the fire that devours and wastes with the fire that kindles, and succours in wanhope and distress; but his joy, and his swift wrath, were veiled in garments grey as ash, so that only those that knew him well glimpsed the flame that was within.
(Unfinished Tales, The Istari).

I do not think that The Ring of Fire was created with the intention of burning thousands of orcs, I think the ring enhanced Gandalf's already inherent power to move people to great deeds and bring them back to hope.