By my calculations (at $10 a point), it always makes sense to reserve weekends in cash at current prices, and for some rooms at some times, it can be cheaper to pay cash on some weeknights.
However, I am still in the market for a smallish (50-75) February UY contract, so we can be assured of VB availability for busy times like Presidents' Day weekend, or Easter, or Memorial Day. The maintenence fees are well worth it for us to be able to have home resort priority during those periods. But we wouldn't buy VB points to use in the off-season; there are too many better deals for those times (Disney Visa, Birnbaum's, even rack rate at some times). But for home resort priority for peak times, that's why you want to hold onto VB points, IMO.