They do add stability to the bow but can often hide improper form.

I have seen archers who shouldn't be drawing back 70#'s add a stabilizer to do exactly that. Stabilize their shaking. Why not just back down a few pounds on the draw weight and not add any additional weight to the bow.

For vibration, I recommend an STS over a stabilizer.

For as inexpensive as they are, it is always a good tool to have in the box whether you use it or not.