Duffey is a waste of time.
He's on the 40 man, he goes through stretches where he is successful, and he has options. Just because he - or anyone else - is not Mariano Rivera doesn't mean he is a waste of time.
Go look at the 40 man and see who should be up here, under the assumption that they wanted to go to 13 pitchers for reasons we don't fully know. But work from that assumption. Also, consider injuries. Also, take a look at recent performances. Considering all of that (and anything else you want to consider), who should be there instead?
One thing I really like about this front office is that they seem to be making good use of the 40 man and players with options. No team has 40 superstars on their 40 man. You have to make good use of the resources you have, and Duffey is a resource.