Reason #1 (the valid one), the tire is being used by a top level pro for whom that last 1% of grip really is necessary. (The rest of us, wouldn't know the difference and aren't fast enough for it to be an issue.)
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And you can add the difference between todays advanced supersport tires to street riders buying dedicated track only tires for their bikes.
Again, quoting a thorough test by BIKE mag, unless you are a pro racer or an extremely advanced track rider extracting and exploring the absolute limits of traction, with todays advancements in tire tech, "super sport tire is fine for the rest of us."
In their test, a "pro" rider was only able to lap the circuit, if I recall correctly, about less than 2 secs quicker than the supersports. (it was a Dunlop Sportsmart vs Dunlop D211 test)
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