^That's a crutch. I think Kirk would have been more torn up by, say, Ensign Garrovick's death than he would David's (something they did in the novels as a throwaway during that one idiot's reign, which pissed me off more than just about anything else in any novel), because he doesn't know this kid from Adam and David doesn't really show him a whole helluva lot in the time they have.
You can hang any particular kind of character change around these family conventions (Spock's half-brother is a perfect example), but that doesn't make them any more legitimate to me. In TOS, it's the family you choose to be with that usually trumps the sperm family, anyway. Band o' Bros & Sis', y'know?