I am so fussy about the fit of the ball, that I feel whatever I would gain by going to a designated spare ball in terms of it's reaction (or lack thereof) I would lose more by the ball not feeling exactly the same in my hand (even when the balls are measured and proven to be drilled identically, I can perceive the slightest variation).
Have tried plastic on occasion for spares. Sometimes it saves me by not veering off target, and others it leaves pins by not having the same drive a reactive ball does.