Depends on what they are watching. If useful training information for their age, it will only benefit. You need to teach them how to properly allocate time and effectively use the computer. And if you just plug them with a computer so they do not interfere, then you do not have to wonder why they became zombies.
In some way I agree with you... it is good for the kids (or any other age) to learn or do something useful (by the computer or any other place), but it is not good if they spend too much time in front of screen, it is not good for the eyes, for the brain...