Should I be catching multiple Pokémon for trading?

[Hernansanz]

Whenever I do a first run of a game, I just catch extra Eevees for obvious reasons and those that I know have different paths of evolution (Poliwag, for instance). When I trade, I mostly send pokes I don't want or the ones I just catch to fill dex. I usually get my extras when I'm breeding, and the ones I don't want I just wonder trade them. 

So I don't see the point of catching extras, at least in the first run