I wouldn't be surprised if some of it really has no effect and that's the whole point. As you mentioned if there's 100 new mutations per baby then if the codin

I wouldn't be surprised if some of it really has no effect and that's the whole point. As you mentioned if there's 100 new mutations per baby then if the coding is as dense as possible that means each mutation will cause some significant change. If it's got a lot of coding that doesn't do anything, by probabilities it'll likely not change anything important and cause no ill effects. Having that extra unused DNA could pose an evolutionary advantage in the face of an imperfect copying mechanism.