I'm not sure we're looking at this the right way.
Blake Sims won the backup job over several guys, which includes Phillip Ely and Alec Morris. This means something, he was backup at Alabama and not merely by default. There were other guys there, that other schools wanted and Sims is the one that not only managed to hold onto that second string spot for a couple of years, but hasn't been knocked off the first string position this year yet either. The coaches don't draw straws to determine that sort of thing.
It's a good thing to be able to return your second string guy once you lose your starter. Not every school gets to do that (they might graduate or transfer). And, on top of that, this is Alabama, not exactly completely lacking in talent. I remember reading something last season that listed Blake Sims as an NFL draft prospect. There are probably plenty of programs out there that would love to have the "problem" of potentially playing Blake Sims at quarterback, a fifth year senior.
On the other hand, Coker is a guy Alabama originally recruited. He's a guy pro scouts love, and he came from a place that's had a fair bit of success with QBs. If you're going to get a backup, you might as well get the backup from a team that just won a championship, right? Coker is the sort of guy that practically every school looking for a starter would have wanted.
So, we can look at it like the obvious fact that Alabama will be playing a guy who was a backup. A guy who has limited experience and has to show some improvement. Part of this is the simple fact that there are some skills a quarterback can't develop until he gets meaningful playing time along with the first team reps. Or, we can look at the fact that Alabama has the #2 guys from two of the top programs in the country. If you're going to have to replace a starter, isn't that the sort of position you'd want to be in? No one wants to start a true freshman at quarterback...