Gonna continue on this cause I think it's a more interesting discussion than anything else on here atm.
I think part of what makes me feel about it that way is because it's always been this way for tier lists I've seen on FE4. The best way I've found to word my problems with giving units credit for good parenting is because we are ranking them in a Gen 1 tier list (or list of unit ratings, w/e). Not Gen II. In addition, parenting in the FE4 world is not something you actively do. All you do is effectively build an FE GBA support (including the support bonus, which I do believe counts as a benefit), but Arthur getting an ORKO on random enemies isn't something Levin is doing - it's Arthur.
If you were to continue on this train of thought, to stay consistent with the logic, you need to consider a child's contribution to a gen II playthrough for their potential parent. This should make Ayra and Brigid drop further, since none of their kids really does anything for an efficient playthrough. It should rise up Deirdre, since Celice is the cornerstone of FE4 Gen II efficiency. It should simultaneously send Lex up and down, since he has so many different potential children. Hell, do you only consider the good or the bad pairings?
The huge logical leap you'd have to make for this to be consistent is just not worth it to me.