Well, backup units are only good if you are doing almost as well in them as you are in your other subjects. If you are doing significantly worse in your backup units, then there is no point in keeping them. For me, I had SOR I as a backup and even though it didnt count, I dont regret keeping it (I think it was close to counting). Ultimately, my mark in it was good enough to the point where if I did screw up one of my subjects, SOR would have been able to cover the damage.