A simple answer to junk DNA

This is not a complete answer, but it is one of the elements in the solution, IMHO. As strange as it may seem, the excess of DNA could serve as a governor or throttle to the rate of growth of a cell, to enhance stability, complexity and control. A virus replicates quickly and so it sees a great deal more variety or randomness in its outcome. A slow process averages the flaws and leads to a smoother and more predictable result. The duplication of elements also allows for averaging of effect and a more consistent outcome that would be necessary for greater complexity. I suspect this is true, and I will bet 1,000 milli-cents that I am confirmed in this hypothesis within the next 40,000 years.