What's the point of oil grades like 5w-30 or 10w-30 or 15w-40? Why not just use 0w-30/0w-20/0w-40 when possible? I've read and searched but could never find a definitive answer
So, why not use all 0W-x oils? Cost, mainly. If the 0W-x oils can be made without excessive viscosity index improvers (which do not lubricate, they just take up space in the oil), then they probably work fine.
Where did you get the idea that polymer VIIs don't lubricate?
It is very slippery stuff that does an excellent job of lubricating. The only negative with VIIs is shear stability when lower quality VIIs are used. The highest VI oils, predominately the OEM 0W-20s, actually use less VIIs than many of their much lower VI counterparts.
The only reason 0W oils aren't used across the board is cost.