Depth means basically nothing for the OL. The guys that start usually play the majority of the snaps
That was certainly true under Lightner, but it seemed to be a self-fulfilling thing. He had no depth, so he mostly played the starters, so he had no depth. Johnson has made more frequent use of the backups. Is he using backups more because he has better depth? Or, is he developing better depth because he is using the backups more?
In any case, depth is certainly of value even when starters play the majority of the snaps. First, linemen do get hurt. What do you do then, if you have no depth? As an example, Sherman, today I've set the way-back machine to October 8, 2005. After a promising start, with a win over Pitt, and a 1-0 record in the MAC, three Bobcat starters go down to injury in one five minute period against BG. With no depth, the season goes south in a hurry, and the Bobcats lose 5 of the last 7, unable to do much offensively.
The other reason why depth is important is for maintaining continuity from year to year. If you want to be successful, year after year, you can't constantly be rebuilding every year on the offensive line. You need to build depth, so that when players leave, there are players behind them ready to step up. That was something Lightner was never able to do as well as I had hoped, but I'm optimistic that Johnson will finally accomplish this.