You don't have to have an entire bullpen of guys who have to pitch every night, but the Cubs pen is in a weird place where you can only hop so many landmines at once. And I say this as someone who's been pro-Miller.
1. He got bombed on his second go of a back-to-back and he barely pitched the first game (one hitter).
2. If you need to pussyfoot around Miller's usage, it helps if you have a bullpen you trust. It's pretty hard to avoid Miller on back to backs, while also avoid Caleb Thielbar or Trent Thonrton outside of curated spots, while also having three longmen who aren't really leverage arms, while also figuring out when to throw Zef and Palencia...
It's a quagmire and right now, it's probably best to give Miller a few more days while you hope a few of these guys right the ship before you bring up Steele and Miller who will add in a lot of other usage questions.