Siping basically increases the number of edges your tires have in contact with the ground. It is good for improving tire performance in water and snow. However, when doing stuff like rock crawling, the edges just make it easier for the rocks to tear off parts of your tire.
...so I'm told.
IMO - I think if siping was the best thing since sliced bread, every tire would have it. Snow tires for example have insane siping, but serious offroad tires have very little (if any at all). If you want to improve your on-road, all weather performance, I think it is a good idea. If you're taking the wheels offroad, I don't know if 'aftermarket' siping is the best solution.