There is no shortage of questions here that relate to stopping or limiting computer time and game playing. What studies are motivating these concerns? I ask this as someone biased because I spent a lot of my youth on a computer and playing games and I am now a successful professional with a family and a good sense of balance (no 30-hour marathons anymore!) and while my parents were usually concerned, they never did anything to limit my use. I had a computer in my room.
I don't see why reading books is superior to playing on the computer personally, but I'd be interested to see some scientific, preferably long-term studies that establish facts about what constitutes abuse or addiction or otherwise unsafe levels of computer usage.
It seems a lot of people ask, "All my child likes to do is [x], how do I get them to stop and find something else that they will like doing?" When they really mean, "something I like[d] doing".
Note that monitoring/restricting what they do on the computer makes sense to me, e.g. white listing domains.