Unless you bowl on a sport shot, you don't know how much the house shot helps with spare shooting.
As a rightly, going for 10 pin, on a houses shot, it's hard to throw it into the gutter. Pull the ball a little and again, the oil saves the shot.
For this reason, I actually thing a weak reactive or urethane may often be better than plastic on house shot.
On a sport shot, you have to be a lot more accurate to hit that 10 pin. Then you also have to have a spare game that will work on 45 feet of oil, or 32, plus won't change when the lanes become toast.
At that point, someone has to be damn good at flattening out their wrist to be that accurate with reactive on so many different conditions.
Some urethane's won't hook at all unless you have some hand. I used to use a Supernatural for spares. Pitch black on other hand, may want to roll at your feet if the heads are dry. I don't think there is any advantage using that for spares over reactive.
What I think it really comes down to is having one less ball to carry. And if going to a tournament putting that plastic spare ball in your bag may mean you have less one less reactive to choose from.
However, when it really come down to it, plastic will always be best. Which is why almost all pros, use it for spares, or Storm Mix, which is hard urethane and pretty much equal to plastic in hook potential.
