The Whole or the Sum of the Parts?

I often reread Revenue Ruling 59-60 just to see if I can gain any new insights. In my latest reading, I was struck by its emphasis on valuing individual estate or gift assets and the fact that it says nothing about valuing a the assets as a collection or portfolio. This is because the valuation is at a point in time (date of death or gift) and at any point in time values are fixed, and their movements over time (correlation or covariance) are irrelevant. Even though a diversified portfolio might be less risky than a single-asset portfolio, this is not considered in fair market value.