Ah but that is different!!
I would say that computer time is exercising-the-brain time. It is all well and good to exercise the body, but with an inert brain it doesn't do you much good. So you need to dedicate a portion of your time to maintaining your body - and a portion to maintaining your brain. And the computer maintains your social / emotional health too. So it does double duty.
For example sudoku would exercise just your brain - but figuring out things on a forum exercises your brain *and* provides a community emotional boost. So it's multitasking
Now if you did computer work 100% of the time and body work 0% of the time that would be an imbalance. But I do feel healthy computer time is good for you!