School of thought has typically been that the narrower a spread of viscosity, the less VII and other additives are used and thus the better the oil will perform over its life.
How applicable it is to any/all syn oils it is not clear. Some likely do use VIIs, some do absolutely shear down, but they arent necessarily controlled in their design the same way as conventional lubes.
GM, IIRC, was the ones that determined that the slightly lower viscosity at reasonable temperatures had the potential to provide slightly reduced fuel consumption at "cold" starts.
The consideration where balance is key is startup temperatures. If you live in a really hot area, a multigrade may provide no benefit. Look at the viscosities at 40C for a straight oil versus a multigrade. The other side of the coin is that if you make a lube such that it is workable at too low of a temperature, the intermediate viscosity at other relevant temperatures may actually be higher than one with a smaller spread. Would this be a major issue? Perhaps not, but given that we like to split hairs over viscosity ratings, etc., it is indeed a consideration!