Based on conversations with many people including those with kids in high school who have spent a lot of times on screens and my own research and observations, there are two big reasons I limit the amount of time:
1) Loss of will to do other things–They become unmotivated to do different activities because they are so entranced by being on the computer. We know one family whose son is in high school and won’t do anything at all but screens. No Scouting, no sports, he can’t concentrate to read, etc. So she has gone cold turkey from screens and is in a Waldorf school with her elementary-aged boys.
2) Psychiatric problems with ADD, ADHD, depression and anxiety. There are so many links here. Also, if someone told me I had a child who needed to be on psychiatric drugs then I would go cold turkey on screens and spend lots of time outdoors with lots of exercise before going this route. I would try this for several months. I have a family member who has been through hell with these drugs. Over the long run, the side effects can be devastating for the person and the family.
Overall, I am happy to let my kids have some screen time per day.They earn it just like a person earns a paycheck. It motivates my son to get his work done while my daughters are not really interested. If you ban something altogether, then they may go the opposite direction when they have the ability to do so.
I do not buy the argument that videos and screen time are good because they help you to understand technology. I agree there is excellent technology to enhance work, learning, etc. Lots of fantastic stuff, and I embrace that. However, sitting around playing video games is not an effective tool in teaching one to use effective technology. On the other hand, strategic use of technology has great benefits. I’m teaching a blogging and 21st-century skills class at our homeschool co-op in the fall. We will incorporate some strategic uses of technology.
Here is one article of many describing effects of screen time on kids’ brains:
thecontemporaryhomeschooler 