Don't worry about Serial Code units. The only time you should care is when they give out free Gold units such as Monica, which are more useful to have than Sapphire units. Sapphire units take too much resources to raise (and that includes skill levels for their skills) and, even then, aren't outstanding units in the end-game level. They're fine if you want to replace Silver units, but they won't be replacing event Platinum units nor are they replacing good Gold units.

Simply put, they're vanity units. They're only for show.