How so?
The changes are hardly radical...
The Brumbies will still be fielding a superior pack on paper, and then in the second half they have the option of bringing on the likes of Alexander, Kimlin and Vaea...
In contrast, the Rebels bench consists of...
A couple of ex-Wallabies, an ex-English international, some players with potential, some journeymen and some rookies.
Looking more at those benches, reserve hooker I'd maybe just give to the Rebels. Prop definitely to the Brumbies by a mile. Lock is tricky, as Kimlin should be good (after a shocker last week), and Vaea handy, but the Rebels have ex-England internation Lipman to come on, with Jones going into the second row. Davidson is a bit down on the others, he's a great club player but unfortunately I don't think can provide the same level of performance at the Super level.
In the backs, there's Prior, who is very solid, and Kingi at scrumhalf, who can be a livewire. Then the Rebels have two Wallabies in Huxley and Johansson (who can't be over rated as no-one really rates him). The Brumbies have the young Coleman, who has been electric at times but rather inconsistent and is returning from injury, and Smith, who I think the Brumbies will sorely miss by having a winger at 13 instead of him.
The Brumbies bench is definitely stronger, but I don't think there's as huge a difference you are implying there is. And don't forget, some of that strength is coming from rotating out Alexander for Murphy, Kimlin for Fardy (not a real lock), Vaea for Fotu (unproven for the Brumbies yet and his first game after a fair lay off), and Kuridrani for Smith. The last could really bite them, as Smith has been great this year and that Rebels midfield looks a dangerous prospect with J'OC and Inman. Weakening the team to have a stronger bench is not the best boast in the world.
This is a real banana skin game for the Brumbies, and I think Jake White is either brave or foolish for choosing to rotate some key players for it.