[Nyara]

Also, let's rebember the stat. exp., when you get Duduo, it's without any stat. exp., while Spearow could have helped you along a good bunch of the time (about the half), and he could just developed stronger stats than Duduo/Dodrio. Spearow is not something like Magikarp that you need to train in order to use, Spearow is useful from the moment you catch it, and even if you don't want to expend to much experience on it, you can still catch one at higher levels, but still earlier.

Just for refreshing the memory, EV = The Square Root of your current Stat. Exp. (rounded down).

It means if you used a Spearow and you wanted to change it for a Duduo; you'll have a worst Fearow always, because it will be (a lot) less trained, making your Fearow trained from those level 4 Spearows a harder hitter (with more speed and bulky) at the same level as a Dodrio. That's an advantage of every each Pokémon you get earlier at low levels and can help you on the while.