We don't need offense from the 5, just defense and rebounding. That would still be true if Facey was playing at the 5.
Facey at the moment is a 4 only. We have to assume he is being developed as a PF, groomed to replace DD when he leaves or moves to the 3. We have to assume that the coaches think learning both the 4 and 5 spots is too much for him at the moment, or that he is not strong enough to play the 5, and at the 5 we have three players (Nolan, Brimah, Olander) plus Giffey when a 4th is needed already.