Great info and opinions here. Really seems to be a do I/dont I need. I have never carried a spare ball with me but have noticed how many do use them and I wondered if there was a real advantage to having one. Thanks for the advice and opinions.
There's the factor of using a strike ball (they're getting more and more expensive PLUS they need more and more maintenance to perform consistently) for a spare ball.
The more you use it, the more maintenance it requires in both de-oiling and in surface maintenance. Throwing it cross alley at many spares, it rolls over and absorbs more oil than when just used as strike ball. Many of these heavy oil balls are sheer oil vacuums.
Surface is always more worry than de-oiling. Using the strike ball for 30 frames is possibly as many as 60 times plus warm-ups per league session. That's a lot of use. You'll have to refresh the surface after every session, instead of may be after every 2 - 3 sessions.
With plastic or urethane, you virtually never have to worry about how or where to throw it. Forcing the strike ball to go straight is not always as easy.