Siping WAS common practice for tireshops back in the day (when I was a kid), ie before tires were modeled and computer simulated blah blah..and it is still very useful for truckers running retreads. In fact, the only tire shops around here that do siping, are the same ones that sell tires for commercial trucks.
Also, lots of 4x4 enthusiasts will sipe their blocky mud tires for rock crawling.