I do think there are some advantages to setting the cutoff point just slightly later than birth, even if by just a few hours:
*evaluations of whether a person should come into existence can rest on surer information when the infant is out of the womb
* non-maternal reproductive autonomy - under the current legal personhood cutoff, I can count this as an acceptable loss, as I consider maternal bodily autonomy and the interests of the child to be more important, but with infanticide all three can be reconciled
* psychologically, parents (especially fathers) might feel more buy-in to their status, even if almost none actually end up choosing otherwise, and if infant non-personhood catches on culturally infant deaths very close to births might cause less grief among parents

(All this assumes that late-term abortions are a morally acceptable choice to make in their own right, of course, rather than something which must be legally tolerated to preserve maternal bodily autonomy.)