I have used both the 50A receptacle to 30A trailer (when I had a 30A trailer) and the opposite - 30A receptacle to 50A trailer. Usually I've needed these when either the needed receptacle wasn't working, but in some cases, it wasn't available. As I can run all but the 2nd AC unit on the 50A and draw only 24A or so, the 30A works pretty well for me when needed.
I have a 30A generator, so the adapter is useful there as well.
I also have a 15A -> 30A adapter; I can charge the mobile house batteries while the rig is parked at the fixed house. :)
You never know when you'll be in a "situation", so flexibility is a good thing.