Aside from Baby Toad, I agree that the babies are rather redundant characters. The only exception in this case; however, is Baby Mario and Baby Luigi. I feel rather sorry for the baby duo as both of them were subjected to the hate that most people showed towards the baby characters due to Baby Peach and especially Baby Daisy since Mario Kart Wii.
Baby Mario and Baby Luigi were fine originally as even though they were not the most inspiring characters, they were still unique on their own in the past. They represented a unique element by being representatives of the Yoshi's Island series. Both babies served notable roles in these games, and it was a nice throwback to see them in the later Mario spin-offs. Both of them also served a notable role in M&L:PiT where they became even more lovable as a result of their heroism.
With only the two of them, nobody complained about their inclusion in the games. Many were fans of them in Double Dash!! and later Mario spin-offs. However, it was the inclusion of more babies in the later games that I believe caused more people to fear not only of the newer babies, but also of Baby Mario and Luigi themselves. I think there is probably some belief that people think that the inclusion of Baby Mario or Baby Luigi in a game now will lead to the inclusion of more babies. This is why I think that both babies started to become disliked recently. I personally like the two; however, adding more babies after them was a rather pointless move in my opinion and did nothing to help the popularity of the baby characters.