While I breed sometimes (like to get the baby Pokemon for my Pokedex), I'd much rather evolve. I like having a lot of babies (I liked GSC's number of babies more than RSE's...which was only three!), but not more than, like, ten to fifteen or so.
Plus, the game has more of a focus on evolving than breeding, so I think they'd rather have you catch a level 3 Caterpie and evolve it up to a Butterfree than catch a level 10 Butterfree and breed a Caterpie (then evolve it into a Metapod).
I think the only reasons they used the Pokemon they did were because they had the sprites ready, they were able to illustrate the different kinds of male/female sprite differences better than the other ones, or they are popular and would draw attention (since it gives reason to believe that they may be in the games' catchable Pokemon). I don't think it necessarily means that you can catch Butterfree or Scizor.