There is a trade off. If you don't swap your people around then they become very one dimensional. Sometimes it better to put someone less experienced in an area on a project so that he gains perspective (ie. see all junior staff)
A good point. A better phrasing would be "you can't move engineers around like that and expect them to be instantly productive". The problem is that engineers are viewed as being one dimensional as in "amount of engineering". Swapping for cross training, skill building, or just plain variety is quite healthy.
