Since you are talking about a C, the truly relevant skills (from the way a BB coach will look at it) is the three you listed the starter as being better in and ID where they are the same. Until the starter tires such that the backup will be a better option at C, the coach is going to keep him in there, unless it becomes a blowout. It sounds like your starter simply did not tire enough to result in the coach bringing in the backup much.