I think some of them were good additions, like Smoochum, Elekid, Magby, Tyrogue, Mime Jr., Mantyke, Munchlax. I mean, pre-evos are a good thing for Pokémon that they obviously couldn't put too early in the game because they'd be too OP, so let's say Magmar is too strong for the early game, well, you can catch and raise a Magby instead.
Some others were unnecessary (Pichu, Cleffa, Igglybuff, Azurill). Unlike Magmar, Mr. Mime, or Hitmonlee for example, Pikachu, Jigglypuff, Clefairy and Marill do look like first stage Pokémon and do have the stats of an unevolved Pokémon so I think baby forms for them were a pointless addition.