Here is what I look at in a spare ball.
1. I want the ball to have as little motion as possible, but not an arrow. Just a little move, which is why I would scuff up my plastic spare balls (just a little).
2. If the ball is going to "move" then I want that motion to occur as early as possible. The thought of a spare ball having anything like a skid/snap motion is frightening. When you get around the corners or on shorter patterns, I do not want any residual rotation to cause the ball to bite and miss my spare on clean backends.
3. Longevity isn't the biggest concern since plastic balls are cheap, but urethane is much longer lasting.
4. Only time I worry about my spare ball doing double duty is if I am ever limited in the number of balls I can use during a tournament. Other than that, I will take and use as many balls as I can carry. Also, I feel some bowlers use their spare ball as a strike ball without exhausting all other options. They try to stay in an area too long and force a shot that simply is not there.
After all that, the Rebel Tank will be a great spare ball for most bowlers. Higher rev bowlers may need to adjust surface or drilling, but with practice, any ball can be a great spare ball.
