A big PrVi could negate other spells. One example I gave was a PrVi 25 could, on average negate a lvl 40 spell. It would be safer to have it at 30. But if you knew the form you could, on average, deflect it with a lvl 15 fastcast defense.
Ask Marko how his PrVi super counterspell went in our game. It worked. After he spent 3 confidence points and fatigued himself. A follow up spell would have gotten him. And that was a Mastered PrVi 15 vs a lvl 25 spell.
Actually, it was PeVi15 versus a level 15 Mentem spell, remember (Confusion of the Numbed Will). The Ease Factor to dispell was 25 or something I think. And yes, it took every ounce of effort. I had invented it for Dimicatio as a generic all purpose counterspell, but when I used it in the field I realized that it wouldn't work in tournament. But it did pull my arse out of the fire in a pinch and threw the beta sg (Jebrick) a major curve ball. I spent like three fatigue levels and 2 points of confidence just to squeeze off a successful fast cast counterspell. Yes, there are better and more efficient ways of doing it, but it caught the enemy by surprise and saved our lives. Eventually it would probabbly be wise to learn the ten different Unravelling the Fabric of (Form) spells. But that requires identifying the form. The generic counterspel was also useful in my situation because the opponent has Silent Casting, and there was no way to identify Form anyways.