Yeah the PMU is just horrible.
If I'm going to buy a YBM fare out of my own pocket, I should be able to use the certificate at the time of booking so that I know that the significantly extra $$ will get me a Biz seat, not having to wait until day of departure.

This how I would fix it:

Though I really would like SWU without restrictions (this is the best for customers) otherwise, YBM get immediate Biz seating, middle fares, maybe 1 week prior to departure, and LUT heavily discounted fares day of departure. Why not give your best customers a nicer seat if they are going to go out empty?

Having options is nice, but clearly what they are doing is not going to keep people happy nor motivated to keep all their biz with Delta.