Some people aren't willing to pay the ever-increasing Y, B, M fare to qualify for international upgrades. Also, there are some transcon flights (LAX-ATL) where the upgrade is difficult to come by. Better to use the upgrade cert and guarantee a seat up front versus dealing with the upgrade windows & gate lottery.

I just used two PMUs for an East Cost-Las Vegas trip coming in 2 weeks. Lowest fare was a K fare and I was able to use PMUs that would have otherwise been wasted.