The suggestion here is that the spell is ONLY useful in these very niche cases....which I will disagree with.This is what people really seem to not understand. In terms of the save reroll the most powerful use is when the enemy has a poor save and made a great roll (needed an 18 and got it). In these cases it can be HUGE. But this does not happen often, certainly not often enough to warrant an entire spell prepared at high level.
The fact that silvery barbs is most useful if the monster will have a very hard time passing a reroll is obvious.... but that doesn't mean its worthless if the save is a bit harder. The real question is: when is silvery barbs "not worth the slot", aka how high a save bonus should the monster have before its not worth the use of the slot.