If DL is going to continue set M fares at a few hundred dollars below BE fares, then if they were serious about making the PMUs valuable, they would allow their use at booking for any available seat. Trying to play the "If you pay $2,500 for the same seat you could have gotten for $700, MAYBE we'll upgrade you when you get to the airport, or maybe not" game with the elites is just plain ridiculous. I can see the need for capacity restrictions if PMUs were valid for any fare, but they're not. Everybody gets 4-6 of them, and they can only be used on exorbitant coach fare classes, so the capacity controls don't seem to serve much purpose other than pissing off elites.