Signing BA to a long term deal is going to help our otherwise constrained salary cap. We have alot of dead money on the books this year, and may need some contract help just to get all our rookies signed.
In addition, it gives us two good tackles for the next 4-5 years, which is a very good position to be in. I don't think the coaching staff particularly cares where they line up, and BA won't really care once he is signed long-term as well. The only one who will eventually start to care is Fisher, but he has several years before he has to worry about that.
And, when the NFL signs a new TV contract it is expected to be for quite a bit more, which will raise the salary cap and salary floor. Large guarantee money with a fairly backloaded contract is probably the fiscally responsible thing to do in this case.
Finally, it will make some of the anti-Fish people to have to shut up if we weren't basically swapping BA + 1st overall pick for Fisher.