Except that IS (aka VC) does nothing to help with subject motion - it works by allowing you to handhold at a slower shutter speed than would otherwise be possible. So, three stops of IS/VC at 70mm, on a FF body, means you can handhold down to 1/8 s. If your living room is dimly-lit, a 1/8 s exposure with VC means your furniture will be nice and sharp...and your 2 year old son will be a blur.I am not talking about subject motion. I tried to avoid taking photos in low speed and when my son is moving. You do take many photos in low speed shoot when your subject is not moving, don't you? I really wonder if anyone taking photos only when your subject is moving?
Sure, I take lots of pictures with static subjects - buildings, mountains, etc. I also take lots of pictures where the subject is moving, and I want to capture that sense of motion - waterfalls, traffic, sports/action, etc.
The thing is, when the subject is a person, they're always moving (well, the live ones, at any rate). If you're trying to capture that motion - blur to indicate movement - fine. But, even an adult holding still as they pose for a photo is going to have slight, involuntary movements that will reduce sharpness. Kids are even worse. Generally speaking, you need a shutter speed of around 1/60 s or faster to stop that involuntary motion (you can sometimes get away with 1/30 s or 1/15 s it you fire a fast burst and get lucky with a frame or two). IMO, that is why Canon felt no huge need to add IS when they updated the 24-70mm - it's the 'wedding/event lens' and that means shooting people, and that means 1/60 s or faster, and with a 70mm lens, that negates the utility of IS in those situations (granted, I don't agree - just suggesting that's what Canon may be thinking).
Picture of a kid in action - playing sports, dancing etc....has more meaning then still shooting.
To a point, yes. But usually you want a shutter speed that captures a sharp face (sharp enough, at least) but moving extremities. Since the extremities move fast (relatively), something on the order of 1/60 - 1/125 s usually works well for that. If the face is a blur, too, it's usually a throw-away shot.