The advantages are only for cold/snow/ice/wet conditions. Siping decreases performance for warm/dry conditions because it allows too much tread block movement. This is why slicks give the best grip on a dry track.

However, since most of us in the real world never come close to the limits of a good tire on the dry the advantages likely outweigh the disadvantages. Did the tire dealer say whether siping affects the manufacturer's warranty?

[Edit] Siping probably decreases tread life as well. This is just an intuitive guess on my part with no facts to back it up.

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