Probably easier to fire up the troops to go to battle against "guys who kill their own babies", cuz then you're not just going to war to defend yourself & your people, but you're also going to crush evil people.
Just about all ancient peoples practiced infanticide. The only exception I’m aware of is the Jews. This was regularly mentioned by their neighbors as one of their oddest customs.
What was unique in Sparta was that a state committee decided which children would be raised. Everywhere else it was the head of the family.
Today it’s the “mother.” We’ve advanced so far.
I have read a fair amount of military history of the ancient Greeks. I would say that was not a consideration for going to war in 5th and 4th century BC Greece. And I doubt that it was a consideration when Plutarch wrote his history in first century AD. He probably disapproved of the practice, but there were no wars against the Spartans at that time.
Plutarch did write an essay "On the Malice of Herodotus" in which he critized the 5th century BC Greek historian. This may be the explanation:
According to Plutarch scholar R. H. Barrow, Herodotus’ real failing in Plutarch’s eyes was to advance any criticism at all of those states that saved Greece from Persia. “Plutarch,” he concluded, “is fanatically biased in favor of the Greek cities; they can do no wrong.”Sparta was one of the states that saved Greece from Persia. If you haven't seen the movie "300", I would recommend renting it.