The problem with only knowing the reaction of the ball you are throwing is this: What happens when you throw your super hook monster on anything by soup? It burns up. But that doesn't tell you if the lanes are dry, or just too dry for your hook monster. The opposite is what does your burnt lane ball do when there is even a little oil? It slides forever. But that doesn't tell you if there is a ton of oil, or just more than that ball can handle.
By taking the medium ball that you can throw on a wider variety of conditions, I think you get a clearer read on just what the lanes are like. Usually the medium ball will at least get a little ways down lane before burning up to let you know how dry they are and it will also move a little on the back to tell you how heavy the oil is.
Using a very condition specific ball to read the lanes would give you a bad read more times than not.
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