There are a number of reasons why having Bill Callahan take over the play calling duties is good for both Jason Garrett and the Dallas Cowboys.
1. Bill Callahan had a pretty good record as a play caller for Oakland back in the day. This post by Jonathan Bales shows that Callahan's teams ranked higher in points scored most years than all but one of Jason Garrett's Cowboys teams (2007). Dallas under Garrett hasn't been that bad in yards gained, but we've consistently underperformed in points scored - and points, not yards, win games.
2. Bob Sturm has another wonderful post about how the offense under Garrett didn't work well, and applauds the move. His main point is that we need to run the ball effectively enough to keep the D guessing, and we haven't done that well under Garrett. To my mind, this has more to do with a bad O-line than Garrett's calls, since the last and only time we had a Pro-Bowl line was in 2007, which coincided with Garrett's best year. But I would also note that Tony Sparano was helping Garrett game-plan that year before he left to coach Miami. Garrett hasn't been as productive since Sparano left.
3. As I pointed out in Tom Ryle's post about the move, by appointing Callahan to call the plays, it actually makes the play caller MORE accountable. Offensive coordinators can be replaced much more easily than Head Coaches, as Cam Cameron found out last year when Baltimore fired him in DECEMBER, and then went on a Super Bowl run.
4. Contrary to most of what has been written, this move SOLIDIFIES Garrett's Head Coaching role; it doesn't undermine it. Per point 3, if the Cowboys offense doesn't work, it will be mostly Bill Callahan's fault, not Jason Garrett's. And if the Cowboys offense works better, Jerry Jones won't be thinking about elevating the much older Callahan to Head Coach, because all he will have shown is that he's good at calling plays, not that he's good at running a team. The same goes for the guys on the defensive side -- Kiffin and Marinelli. If they get the D in ship shape, that doesn't mean they can also be Head Coaches. Both are really too old for the job, and neither has any proven worth as a Head Coach. Kiffin's never done it, and Marinelli failed at it.
5. In other words, if this works, Garrett will get the same kind of credit that John Harbaugh got as Head Coach in Baltimore, but if it fails, the onus will be on Bill Callahan, not Jason Garrett. This is a heads he wins, tails Callahan loses situation for Jason Garrett.
6. Jason Garrett can now focus on fixing all things Cowboys, and part of that includes holding the coaches under him accountable for the performance of the units they are responsible for. If those units play poorly, it will be on those coaches, not Jason Garrett. Garrett isn't emasculated -- he's actually now more in charge than ever.
7. What will be on Jason Garrett is the kind of stuff where he's tended to do well. Assembling the roster. Keeping Jerry Jones from reaching for older free agents. Getting the team to play hard. Getting guys who are solid character guys (Josh Brent's serious error in judgement notwithstanding).
8. If the team stops playing, like it did for Wade Phillips, the blame could fall on Jason Garrett, but that doesn't seem likely. First, because there would always be a coach below Garrett who might be at fault. Recall that Wade was the D Coordinator, so he had to take the blame when the D fell apart. Second, why would the team give up on Garrett when they've gone through worse over the last few years?
By moving Garrett to a walk-around Head Coaching position, it makes him more like Mike Tomlin in Pittsburgh, or John Harbaugh in Baltimore, or even Bill Belichek in New England. He can now be in charge of the TEAM and have replaceable underlings take responsibility for everything else. This is actually the ONLY way Dallas could position Garrett as a long-term solution at Head Coach.
Late last year, when I wrote a post on why I thought Jason Garrett should be fired, much of my criticism focused on Jason Garrett as Offensive Coordinator. I also thought Rob Ryan needed replacing. Now, both of them have been replaced. Garrett will still have a role helping shape the offense, but his fate is no longer tied to how they perform from game to game. Garrett's fate is tied to how the overall team does, and that has to be a good thing.