"Microsiping can dramatically improve tire traction in rain and snow. However, microsiped tires may also have increased road noise and tire wear when operated on dry surfaces. ConsumerReports.org recommends against adding more than "the sipes that your tires come with" because of longevity and dry performance"
"Claims that extended life is achieved by siping may only apply to certain environments, operating temperatures, and rubber compound builds."
http://en.wikipedia.org/wiki/Siping_(rubber)
Basically if you live in a place where it is wet or snowy over 50% of the days you drive, it is beneficial to get your tires siped. Thats what I took away from the 10 minutes of research I did. I also read you can sipe your tires yourself with a razor.
Now think about it. Do our day to day tires actually get as hot as these people are saying that they need extra slits to "prolong life"? Maybe if I take my car to the track. But then again... I would have a special set of tires for that. Typical salesmen selling things