Serious video use requires a support system just like some types of serious photography require a tripod.
If you're doing a war reportage, you would benefit more from an IS than from a tripod or a steadicam... that's just an extreme example, the point is that dogmas are always harmful. Even with a professional "support system" like a rig or a camera stabilizer, IS can be very useful. I almost never shoot hand held, but I turn on the IS very often.
People used to argue often about NOT having IS in the body, which was also ridiculous since optical IS is better than electronic IS.
In-camera IS can also be mechanic (sensor-shift). Canon claims to use an in-lens stabilizer to customize it for each lens and get the best performances. This choice has its pros (you see a stabilized image in the viewfinder and, if what Canon and Nikon say it's true, you get better results) and its cons (much more expensive lenses and less chances to have an up-to-date IS system, since you usually change bodies more often than you change lenses).