The point is we are in a win now mode for the next 2, possibly next 3 years. We could sign Jackson for that duration. While it's great to develop our own talent, having a back like Jackson would open things up in the middle and back of the field. Plus, like McDMan said, Jackson has been going against heavy loaded fronts for the most of his career, and his OL was never solid outside their retired All-PRO left tackle.
And for those who say RBs decline after 30, well... he will be 29. Priest Holmes started kicking ass when he got near 30 and into his 30s.
We wouldn't have to worry about running for crucial short gains, and Fox would have the bruiser he likes that controls the game and brings pain to the defense.