There's a psychological aspect to it which you're forgetting. I've got a USB webcam for my desktop, which I leave sitting on my monitor but unplugged most of the time, and I still find that unnerving sometimes, despite knowing that it's not plugged in. Having a physical thing blocking the lens puts that unease to rest.
Also, so what if it's a "little plastic shutter"? It's not like the webcams can see through plastic, and it's likely that you'd want to build the shutter out of the same materials as the laptop chassis (usually plastic). Size has no effect either, provided that it's fully blocking the lens.
Whilst I agree that a hard on/off switch would be best, it would probably cost more to manufacture, and I don't actually think that it would be as effective as resting peoples fears of webcams.