Our entire system is based around a good, defensive center. If we let Favors walk and Gobert gets injured, we are screwed. Are we really going to rely on Bradley? If we let Favors walk, we have to go out and get a reliable backup C anyways. With Favors, we will sleep soundly knowing that we aren't relying on one C or everything could fall apart.
Like I've said 100 times, I don't want to bring Favors back at $18. I would let Favors go in a second for a Tobias or Kemba. However, in the event we don't get a $20+ million quality player (which is 99% chance based on our history), then we have to find a way to keep Favors.
Favors is more than just a back up C. He can play some PF. MOST IMPORTANTLY, his style at C is different than Gobert's and sometimes you need a different style C if the playoff opponent forces changes.