"What is so damaging about tv?"
this is quite a complex issue. You could try reading the book in question? There are many facets and I can't really sum it up in a few words. However, tv is not good for children, mentally, physically, and socially.
In answer to yout other questions: ds goes on the computer about once a fortnight, about as often as he asks to, and we might look at a website together for 10 mins or so. I don't watch tv myself, no. But tv is more detrimental to children than it is to adults.