I don't think that it's helpful to extrapolate from statistics related to TV viewing habits of children who are likely somewhat educationally neglected by their parents, when one is considering the effect of educational use of computers. I've never seen any statistics showing a negative impact of educational computer use (have you)? It seems to me that error may lurk in the aggregation.
My take on this is that immoderation in any form is bad for you. I mean, few people would argue that reading is bad for you, but spending an excessive number of hours with a face in a book limits ones exposure to the real world, and more importantly, other people. Few people would argue that exercise is bad for you, but excessive exercise, particularly the repetitive kind, leads to injuries. Sunlight helps promote positive mood and vitamin D production, but too much leads to sunburn and melanomas. I could go on and on.
All things in moderation... including moderation.
