First, I agree that PB values have a problem. Look at the following two pitchers. Which one would you rather have for the upcoming season in traditional (stats are from RT - Williams is easily outperforming Garland)

Jon Garland,R PB 3.454
5.4 5 ERA 4.51 IP 191.2 OPS .798 .710
Woody Williams,R PB 2.865
5.9 5.6 ERA 3.87 IP 220.2 OPS .714 .692


That being the case, most people want some type of salary cap. We have in place only one way of determining that, and thats PB value. Thus that is what we use.

I figure that over an entire roster things balance out. It should be noted in your example that Lorreta has the advantage over Affeldt in that your example would require two extra roster spots for Affeldt to equal a SP while having Lorreta allows you to have two good young rookies with very low PB values on your roster.

PB has always had a problem with part time players, and hopefully that will be corrected someday.