In theory, yes, a Sonic X can be used as a spare ball, given the correct conditions.
If you are accustomed to bowling on a medium-heavy shot where the Sonic X would just skate the entire 60 feet, then yes, it should be a sufficient spare ball. Also, if you completely kill the hook on the ball to shoot at your spares (like I do with a Flipside Wild, RG and Diff very similar if not the same as the Sonic X) than you've effectively used two spots in your arsenal (dry lane and spare) with one ball.
However! If you see a broken down shot, something that hooks at your toes and doesn't stop, and you choose not to kill the hook, then obviously a Sonic X will not work, a very shiny, polyester covered rock such as a White Dot or a Maxim would be your best choice.
A lot of this seems like common knowledge, but the only stupid question is the one un-asked.
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Andrew Loose
Illinois State University Men's Team